First Person Shooters are a tricky thing for Multi-Touch. I worked out a basic set of controls for RPGs like WoW, but FPSs are very different.
I think I'm getting there though. I'm currently working on a Multi-Touch Keynote video, so after that I'll try to show some of my ideas for Multi-Touch RPGs and FPSs.
If done right I could see FPSs working very well on 13in+ size displays. Not just on small handheld devices. I have found that the iPod Touch FPSs aren't as fun because the screen is too small. 13in+ displays would provide more room for your fingers to work.
One of the most important aspects of a tablet is how it will be held/propped up. That matters a lot when deciding how to use it. So how do you plan on your tablet being held? With the iPhone you can hold it like a traditional mobile console, only instead of hard buttons you have virtual buttons on the sides of the screen. But I can't see someone holding their tablet like that, playing an fps.
One particular reason I don't think a true fps like halo, battlefield, etc. would work is because multi-touch just doesn't offer a viable way for accurate yet quick aiming. iFPS on the iPhone is very fun, but the shots have splash damage because you just can't aim all that well.
Cube for iPhone has a decent aiming mechanism, where you can tap in a small area around the crosshair to aim right there. That works, but to do that they had to substitute a dpad with creative but mostly unusable accelerometer controls. A bigger screen MIGHT solve that problem though.
Anyways, I'm really interested to see what you've come up with.
Nate