Hmm. I'd say some form of Soul Calibur should probably exist - it's good for combo-style in the same way Tekken is and is the classic series for weapons, so usually a good one to give some representation.
As for what to replace, I'll just say whatever's least popular. On the current 4, I only know SF & SSBB, and I've not managed to actually play SF4 yet, let alone SSF4, so no major comments on those, just on what I think should be included that isn't.