I don't really see multiplayer happening on the GP32. It would probably require too much data transfer between GP32s. And over any other medium would be pointless.
I guess the only thing I would like to see is support for WADs that don't use the original Doom engine. I know that would be a tough task, but Eternal III is an awesome WAD, and it would rock on GPDoom.
Acutally, that wouldn't be exactly 9.6kb - which is what I presume you meant to type, otherwise you had a rare 960 bps modem - it's a little lower than that due to data size conversions. I don't know if the GP32 RF modules can handle bitrates of that size or not. But enough with cemantics, I'm most concerned whether or not GP32 has enough RAM/Processing power to handle that kind of information. I'm not that worried about bandwidth per se, it's just that a multiplayer game requires more processing than your average single player game.