I think everyone forgot that there is a ranking system in the 64's MP. That being said, I actually liked some of the 64's MP maps, those should be an addition to the T2 Remaster. I also agree with vis about the LAN, seeing as I play T2 MP with friends over LAN all the time.
But onto what I would really like to see:
The game running on a new engine, being actually remade rather than just a bunch of mods added to the game, which I could have done myself. Seriously, T2 is 18 years old, we ought to get something remotely new. That being said, the gameplay should still be the same. The AI could also be improved, though it was great, it was made to work on systems such as the N64 (Which had something like a 94 MHz CPU!) Many other things have already been said, such as MP Gore, more characters, SP guns and beta guns, etc. I would also love a reorchestrated soundtrack and stage editor. A first person body would also be nice, players could also have dynamic footplanting and other fancy procedural things.