In this context I'd imagine that "more dynamics" == "more samples". You can tease a bit more out by analyzing trigger patterns, but ultimately for dynamic realism you're talking about storing (and streaming) tens of gigabytes of samples. Poor little iPhone would break down in tears -- heck, even a low end PC has trouble doing this faithfully (as we've probably all encountered by now).
That said, there probably is a middle road that can be followed. Perhaps something along the lines of Roland's COSM technology where you start out with a smaller set of samples and then rely on digital modeling to alter the characteristics and dynamics. In fact, I'd be very surprised to hear that Roland, Alesis , et al were NOT working on such an app. I thought I'd seen some apps before that used FM synthesis (aka old skool) which probably makes using an iPhone as a sound source a reality, but you'd never expect high quality from that.
If a tiny portable sound source is what you're after, then the V-Machine (or variant) is probably what you're looking for: http://www.smproaudio.com/index.php/en/products/v-machines/vmachine
If cost is an issue and high-realism is not, then a small Roland or Alesis SR drum machine might be worth looking at. These are also good as back-ups for when the crap hits the fan too, as they don't take up much space in your gig bag.