My only issue with the Tekken + Street Fighter game(s?) is that, I don't want to see Tekken characters in the Street Fighter IV engine. I'd like to see them in the Tekken 6 engine.
Street Fighter 4 has a really strong and well done cartoon look to it, while Tekken 6 was trying for a more realistic representation. I think it would be a lot more visually interesting to incorporate Street Fighter into the Tekken 6 portrayal of character. The detail is stronger and would give a sharp pseudo-realistic approach to the characters, which for the most part has been lacking as Street Fighter artwork has generally stayed on a cartoon-like level.
Of course, knowing the Tekken team (which I really don't) they would probably step things up by incorporating some kind of slashy storylines and possibly bring around another final fight 3D adventure thing, which was a nice touch in Tekken 5 and more fleshed out in Tekken 6. Perhaps even character customization. Which to say the least is speculatively far more content than cell shaded variations of Tekken fighters in a street fighter IV setting.
LOOK WHO CAME: