First up a disclaimer, I do not intend for this to be a wish list topic.
I will also say, I don't watch phineas and Ferb, or gravity falls. But from these forums I know a lot of people want these characters.
I know they haven't been included as of yet, well only Ferb in the first example.
And it's argued that they wouldn't be popular enough to merit the development of a character, and release of a figure.
Now I'll be honest, I no longer understand this reason whatsoever when both franchises recently received a full fledged Nintendo 3ds game each.
Meaning Disney has pumped enough money into developing an entire game based on each franchise.
I'm pretty certain producing an entirely new game, even on the platform that would be cheapest to develop for, surely is more of a risky move, than simply developing a few characters and figures for a game that already exists and is making enough money on its own.
From what I've seen about gravity falls, it seems like some people would buy the figures just to own the figures, without even owning Disney infinity.
I honestly think they could make as much money off of these figures as they could expect to get from releasing a whole new game.
Now from my personal wants in Disney infinity, I'm not too bothered, I will admit there are characters I feel merit inclusion first. But from all the fans I've seen asking, and the promotion lately of this new game, it really does make me wonder why they can't just throw them in. It's only gonna make money, not lose it.