Capcom is currently holding its first location test for the arcade version of Super Street Fighter IV over in Japan, and according to this shot thrown up on Twitter by a user named yoshizo715, the version on display has a slightly different character select screen.
The new characters will probably come with a round of balance changes for the entire roster, which will likely filter down to console owners as a free update. New characters, however, seem like the sort of thing that you'd have to pay for to use at home.
I feel like the new characters have already been hinted at by Yoshinori Ono, the game's producer, but he's been hinting at so many different things lately that it's sort of hard to keep track. So let's just say that the two new characters will be Alan Wake and Lu Bu. Wait, no. Sly Cooper and Shin Akuma? How about Jin Kazama and Zero Gouki? Hmm... Scorpion and Sub-Zero?
OK, R.Mika and Karin. And that's my final offer. We'll have to see if future location tests reveal what these new roster slots are actually for, or if we're going to be kept in the dark until the final game is released to arcades later this year.