Ok, let's get this puppy started. Let me start by collecting and organizing all the thoughts I've had, and I'll try to present them in the most constructive and practical context in terms of actual development possibilities within reasonable financial investment.
1) The ability to map any non-sequential notes to the xfade. I'm assuming this one shouldn't be that hard because it's simply just intercepting the MIDI messages and remapping them on the fly, which a lot of software already does and some are free to download. MIDI OX, eDrum MIDI Mapper, S-Note Matrix are a few that comes to mind.
2) The ability to set the xfade points and the ramp of the xfade, so that depending on the sounds you are mapping, you can control how many sounds you want to map, (sometimes you only want to map 2 or 3, not 4, and maybe you even want to map more than 4) the xfade ramp length, position within the dynamic range...etc. I don't know the flexible the code is for Z4's software, so I have no idea if this is even possible currently or would require significant rewrites.
3) A software editor. This is a big one. You can either use the MIDI as the interface for the software editor, or add a USB connector. Some people don't use hardware modules and work exclusively ITB, so USB may make more sense to them, and cheaper/easier to add USB hubs than to add MIDI interfaces to your rig. The software editor would make changing parameters and navigation a lot easie, and I would think it's also cheaper to implement since it requires no hardware changes. For example, that graphical template image on the site now--it would be similar to that for the main interface screen, and each trigger's blank area would be live input boxes for changing settings.
4) However, with that said, for people who don't like to deal with computers, hardware changes down the line may be inevitable, as more features (and the more comprehensive/complex they are) would require more streamlined and clever ways of navigation. The art and science of UI design is not something to take lightly (as we've seen how the manufacturers who do end up feeling the negative impact of the lack of attention they pay to the interface of their products, and those who do--like Apple's iPod or Novation's ReMOTE SL series, end up leading the market). The Zendrum in its current iteration is already suffering a bit from clunky navigation--too many button presses and too many horizontal selections in each tier of parameters. A couple of well-chosen dedicated buttons, knobs, or data-wheel can greatly improve the navigation. This is a double-edged sword because the original design philosophy of Zendrum is to set it and forget it and just play, but I don't think that philosophy is as applicable today, as the landscape of music-making tools have changed with computers and plugins. With so many choices for drum sound sources, the ability to tweak your Zendrum's calibration, layout, and xfade note mapping...etc would be frequently used, and with frequent usage, the "annoyance level" of a user's navigation experience must be minimum. The least number of button pushes, the least number of directory jumps...etc as possible, to get to often used parameters.
Also, to help along with more intuitive navigation, a LCD screen that can display more comprehensive data would be very helpful. Cryptic display messages are the root of many UI navigation evils, and often when you haven't consulted the manual for a while, you forget what the shorthands stand for, and while on the road or in a gig, you don't have time to consult the manual (or even have it with you). Concise and clear LCD messages will alleviate that problem. Investment wise, this may be one of the more costly ones.
5) The MIDI in/out location/angle is awkward since they jut out at 90 degrees angle from the body, and if you aren't careful there could be cable strain problems. It would be awesome if the in/out sockets themselves are angled and maybe rotatable so you can swing them towards the front or the back depending on your playing position (I haven't seen anyone do this yet on the market). Or, maybe design a simple right-angle MIDI adapter that's like the right-angle 1/4" adapters, but for MIDI cables. This shouldn't be too expensive to implement/design.
6) Factory default preset layouts for popular drum libraries or modules would be awesome, so people can plug their new Zendrum in and start rocking out instead of trying to figure out how they want to map everything. Increase the number of presets, and add additional banks for the presets. For example, melodic scale presets would be in the scales folder, and software drum libraries/moduels would be in another folder, and hardware modules would be in another folder. Of course, the drum layout presets are going to be subjective, but I think with the benefit of experience on the side of the people who are doing the mapping, their layout would tend to be more user friendly than someone who's never spent time on the Zendrum, as they've experimented with different layouts for a while now. This one only takes time but not much investment. Also the Zendrum community can contribute to this particular project with their custom layouts.
7) The manual needs to be clear and intuitive (I know it's being worked on as I type this), and especially the complexities of calibration and how the three current possible methods of calibration correlate to each other, and describe what method is recommended for particular situations (for example David, the example you described on the phone of the PA/sub woofer vibration setting off some of the trigger--that's a great example of one situation). This takes time to get right, but not much investment.
8 ) While calibrating, we must have clear visual feedback of which triggers are actually active, since it's too easy to activate the wrong one--a rub of a shirt sleeve against a trigger you didn't intend to activate, or while setting the max of a trigger the vibration causes a nearby very sensitive trigger to be activated instead...etc. The LCD screen can really help with that. We can have a small graphical representation of the zendrum and all its triggers on the LCD screen, and the activated trigger would be highlighted. Another approach would be to have a tiny LED light accompany each trigger, so whichever one is activated, the light would be on (or maybe blink). The previously mentioned data-wheel can also help with the accidental setting off of unintended triggers--for example, scrolling the data-wheel you can go through all the triggers and the LED light would indicate which one you've selected. Now, add the ability to have the LED light be different colors--now we're talking even more effective navigation. This can even be used to identify what notes are being mapped to which trigger--for example, a melodic instrument--you click on the note in the VSTI and the corresponding trigger's LED would light up green. This could all work in conjunction with the software editor, where there would be a much larger graphical representation of the zendrum and its triggers and what is mapped to each. This one shouldn't be too expensive to implement since it's just low-tech LED lights.
And now a few suggestions that are not hard-ware/software related (these are just some subjective ideas--feel free to ignore them if they contradict the mission statement of Zendrum as a company):
1) Make some really good video tutorials about how to use the Zendrum. What is on the website and in text is quite dated in both presentation and organization/accessibility. Today's instrument/software developers typically all make very helpful video tutorials for their products, and their knowledge base/FAQ...etc are usually laid out very intuitively and attractively. This one might cost some money if you farm it out, or if you do it yourself, will take time.
2) Better presentation/marketing overall. Now, this is not about growing bigger and spending obscene amount of money on ads--it's about utilizing what is freely available to you and strategically placing ads in the most effective places (online communities and specific magazines). Make a really comprehensive and compelling video promo that gives the Zendrum the prestigious presentation it deserves. Beautiful photography, compelling copywriting, wonderfully directed video promos...etc. Put the videos on youtube, put the beautiful new photography on the site, and strategically buy a few ad spaces in places that will really make a difference. Stress the difference between Zendrums and conventional rubber pad based products, edrum kits, and keyboards. Demonstrate the things that Zendrums can do that those input devices cannot--not user home videos but professionally produced videos that are compelling, entertaining, educational, visually beautiful to watch. Contact the people who are behind popular musician's podcasts and online portals and invite yourself to be interviewed or have articles written about the Zendrums. Give tours of the Zendrum facility to the people who are behind the press for pro audio news and they will help spread the word. This could either cost you a lot or very little, depending on how you approach it. Local video production talents may not cost a lot, and free press won't cost you a dime, if you know how to get it.
3) As wonderful as Zendrum is, many people who lust after that kind of responsiveness from a drum input device can't afford one, and may never be able to afford one. I know it took me many years to finally get one, and there are countless others like me. MAYBE (and this is a big maybe, as I do understand that some companies want to remain small and would never ever consider becoming a mainstream player as it goes against their values and goals, but if not, read on) there could be a separate product line that utilized the same design approach, but can be mass produced. Instead of wood, use a durable aluminum shell or hard plastic, as long as it does not change the way the triggers respond. I can guarantee that a Zendrum line intended for the mass market will dominate all the drum controllers out there, as Zendrum's design is obviously far superior. I'm assuming Zendrum has a patent on its design, so it'll be able to lead the market without blatant imitations. Sure, big boys like Yamaha, Roland, Korg...etc will want a piece of the pie and come up with their own competing line, but they will never go as far as the premium line of Zendrums--the hand-crafted wood models. So, in a way, it's a win-win situation, as you've allowed more people to be able to enjoy the responsiveness of Zendrums, while maintaining your line of premium models for those who crave that handcrafted feel of a high-end instrument. This could potentially transform Zendrum Corp. as a company, and some may argue that it's not necessarily a good thing. I personally think that there is a difference between selling out and expanding one's market reach and bringing more joy to more people at a more affordable price, while maintaining a premium product line. Look at Parker Guitars--that's their business model.
Anyway, these are just some of my suggestions off the top of my head. We can talk about the nitty gritty technical and creative ideas by private email as I suggested on the phone.