The last cutscene in Sonny 2 is just another way of saying: "Will be continued." It's still too much unrevealed information in the "Sonny world" for it to just end there. Well, we now know what's on that tape, but we do not know what it means. We also don't know Felicia's intention of stealing the tape. My conclusion is that Sonny 3 will come, but it will take it's time.
Well, there will be hopefully newer zones and achievements on the way. I prefer newer zones than a whole new game, I mean, why bother? Taking the slightly easier route makes much more sense...