This is a difficult question and an excellent subject to discuss.
Basically, we all think differently depending on what we want to see in a fighting game. Some prefer gameplay heavy based on combos (MvC, as you have discussed), whilst others prefer mindgames, pokes and set-ups (SF).
To me, Smash is fighting game that is characterized by its freedom and stage control. To implement this in the gameplay, I suggest that the mechanics will be somewhere between SSB64 and Melee.
To evaluate, I really like the extended hit stun that we see in SSB64, it makes for some really interesing combos. On the other hand, that game is solely based on the "don't get hit" principle, which can evoke anger even to the best players. Melee, on the other hand, has less hit stun and a far more superior DI, making longer combos more difficult. Good DI enables the opponent to affect the situation, which is good, because that makes 0 % death combos very hard.
What I don't like about Melee is that at the lowest percentages, it's too hard to any combos at all. This means that you are forced to use a limited part of your moveset in the beginning. Doing an aerial, tilt or dash attack when your opponent has 0 % will almost be punished.
Also regarding mindgames and pokes: the tilts most be tweeked. In Melee and Brawl, they are, sadly, not so useful. I like the Ftilts that Fox (SSB64), Dr. Mario and Dedede have, because they are excellent tools to poke, and Pikachu's Utilt in SSB64 for combos. Luigi's Dtilt in SSB64 is great too, both for combos and mix-ups.
Like I said, somewhere in between Melee and SSB64 would be ideal. Keep or alternatively expand on the Melee DI and leave the hit stun prolonged, almost like SSB64. That way, we can have some interesting combo, while mindgames and pokes get a more important role.