I don't think Mega Man used a recent design but rather followed his NES-look, so a similar thing could happen to Pac-Man... or maybe he gets an original design for Smash which mixes up old and new? Then again, it's a ball with limbs, so it'll look pretty much the same anyway hahaha! Except for maybe the facial expressions.
Nowadays I don't really care what design they're gonna use for him. I've grown to like the Ghostly Adventures one too (which I really did hate at a point!). I just want him in the game!
Mega Man, as most the characters in Smash, is using his most recent design/incarnation; Well, in the practice, here's the last Mega Man game (canon):
Now, another thing.. the redesign (actually, some touches in his basic look, in fact) was made by Capcom, and not by Sakurai,... and probably, not only Sakurai liked the idea of a NES-alike Mega Man, but Capcom prefers this way too... Now speaking about NAMCO Bandai, as we talked here, the current target of marketing is Ghostly Adventures, and they're putting Pac-Man this way everywhere they can (well, it's not something strange, after all, this was an entirely reboot, ... previous versions not "exists" anymore, kinda), so, besides to be pretty confusing change his design just for a ocassion; would not so smart for their part... specially considering how popular Smash is,.. keeping Pacster how he is nowadays for the game would help a lot his image.
But I'm sure that Pac will please all the tastes even with his modern look,... just taking Sonic as example. =)