They could build off of the T6 or TTT2 system but I don't see how it could be a similar replica. There's two problems:
1. Tekken characters have huge command lists. Trying to come up with similar new commands and motions for a whole bunch of SF characters would require a lot of funds, time, and work. It's not the same as translating Tekken characters into a SF environment.
2. Projectiles are mostly useless in the current Tekken system. They'll need to find a way to make them work for SF characters rather than just be useless additions.
Both of these problems would require them to come up with a new system that builds off the old but accommodates the new. At least, that's my speculation. Who knows how it'll turn out.
Well, thinking about it. If they go about trying to just stick the SF characters into the TTT2 engine, that could be boring maybe, but I think that is the route they are going to chose.
About tekken 4, I am thinking if they tried to play off that, with uneven terrain and the push system, maybe they could do something interesting with having more varience in combos that work on different positions in the ring, and using the push system to get positional advantage.
Also they might take from SC5, and add more flash to the specials, have a limited special meter and stuff like that.
Can't wait to see what they got cooking. I hope they are making it for next gen hardware as well.