Well, it isn't until 3.0 comes out that hardware addons could be compatible with games. So it's only now finally possible.
Whoever makes the iphone's first gaming addon, three requirements are must haves...
1. The peripheral should look very sleek, flush with the phone and use the same black plastic found in the front of all iphones. iPhone owners care greatly about how thier device looks.
2. The peripheral must have dual analog sticks/nubs. Imagine the PSP's analog nub, but on both sides. Portable gamers have been clamoring for this feature forever now, so it's about time they get it. I can't overstress how important it is the the iPhone's gaming pad has DUAL ANALOG NUBS (and also shoulder trigger buttons).
3. The peripheral must have two shoulder buttons and a solid D-Pad akin to the PSP/PS3's D-Pad to enable fighting games.
Do these things and the peripheral will be a big hit.
Also the smaller the add on, the better. It should be as small as humanly possible. People don't like large bulky addons. And it should have a rectangular design with curved edges identical to the iphone.
I want to see this prototype...
http://www.flickr.com/photos/danielmacdonald/2127089390/
If anyone can pull that off and get a lot of support for such a peripheral, it would be apple.