Out of the *big three* Third-Party characters that everyone capable of logical thought is expecting, Sonic is the big'un. If Nintendo is revealing each of these characters before the game is released (and they will, in order to maximize sales) then the obvious order would be Snake, MegaMan, Sonic.
With this in mind, I'd say it is much more likely that we see MegaMan tomorrow at E3 than Sonic, if any new characters are shown at all.
Another reason I'm sure all three will be revealed before Brawl is released is because Nintendo will absolutely want to put Snake, MegaMan and Sonic on the boxart for the cover of the game. More sales.
With this in mind, I'd say it is much more likely that we see MegaMan tomorrow at E3 than Sonic, if any new characters are shown at all.
Another reason I'm sure all three will be revealed before Brawl is released is because Nintendo will absolutely want to put Snake, MegaMan and Sonic on the boxart for the cover of the game. More sales.