I'd like to add my voice to this. I've been recording DAW-style since '99/2000 when I started with Pro Tools, moving from ADAT, and tape before that. When EZdrummer came along it was a godsend for working on a computer DAW. Years have passed and I'm appreciating recording on the iPad more and more. Auria is a wonderful DAW platform, JamUp is a tremendously great sounding guitar sim. I've come to appreciate a minimalist setup. I now have a Blue Yeti and an Apogee Jam, and between those two tools along with the iPad and Auria, I've having a great time putting together demos.
The only missing piece is drums. I'm using Drum Loops HD, which is actually really good, but EZdrummer is obviously a different animal.
Some flavor of EZDrummer for iOS would be an incredible tool. I do understand the limitations that come along with using iOS vs Mac OSX or Windows, but a version for iOS that is somewhat stripped down and simpler would actually be welcomed. Toontrack already has a mother-load of samples and grooves already decked out. So it would basically be porting a smaller portion of that content to the new platform and interface.
As someone else mentioned, Apple's in-app purchase scheme does seem like a good fit for Toontrack. Toontrack is the king of add-ons and expansions, all while keeping everything top-notch quality (not an easy task).
I'm hoping a Toontrack iOS drum app is being worked on as I type this, or at least being actively discussed/planned.