One way to make SNK fans happy and Cap fans happy is to have controls similar to MvC2 in the fact that there are four buttons for hurling moves and two buttons for assists...
Which might bring another idea: how about different grooves for different play styles?
For example, a tag groove would use the six buttons in a different way compared to a ratio groove. The tag groove would have four action buttons (that would make SNK fans happy) and the ratio groove would have six action buttons (to make Cap fans happy).
This necessitates the need of a random groove...
My idea of "tag grooves", in which new characters would switch in by a jump kick and then their intros (which of couse taunts)
Cap - MSHvSF style, two characters, three levels of super (one to start). Sakura, Zangief, Orochis, Gouki, Rugal, Kyo, Chun-Li could get "mode change" supers that cost one level of super that would give them a different set of moves. Four button play, with the other buttons being "tag" and "assist". Tag+Assist does a team super.
Cap - MvC2 style, three characters, five levels of super, no mode change. Four button play similar to MvC2 (with the delayed supers)
Cap - MvC1 style, two characters, super bar only used for duos. Four button play similar to MSHvSF style mentioned above, Tag+Assist does duo.
SNK - KOF2000/2001 striker style, 4 characters in any fighter/striker capacity, mode changes. Four button play, and summoning strikers would be done by pressing the other two buttons simultaneously and, if necessary, holding a direction. Counter/Armor modes started by pressing three buttons in a line. Super bar used during striking if striker performs a super (ie. you briefly control the striker when you call it).
SNK - early KOF striker style, 3 characters, any of the remaining characters that have not fought may strike, super bar used during striking if striker performs a super. MvC2 style controls.
SNK - AOF spirit meter/duo battle style, 2 characters, 2 super bars. AOF 3 button controls, both characters fight at once. All moves use various amounts of super bar, and it must be manually charged by pushing all three of one character's buttons.