I believe that Smash Bros. Brawl will be online with at least 1 vs. 1 humans with all/most items enabled 100% going to happen.
I am not so sure about 4 player verse mode...but I am willing to bet that is 100% possible with limited items. I am thinking Assist Trophies and Pokemon may be out, but guns, health, melee weapons, ect will be in the online version.
Also I can see trophy trading, and potentially the ability to unlock secret characters that your friends have on their Wii systems.