The Danger team already accomplished what you want with the sidekick. Word on the street is that Danger 2.0 (android) was acquired by google a few years ago. Everyone I know who works at google (except the lame people) hate it. Maybe you can figure out how to poach all those people.
Getting anything interesting onto an existing carrier's phones is a nightmare. Not only will you need to make a better phone OS, you will have to start your own MVNO. Helio has made an attempt - their "ocean" device might be worth looking at.
I used to work on Razor software. I knew it was junk, but where to start fixing it? There is a deep problem with the companies that create mobile phones- carriers are their customers, but not their end users. The only company that seems to be innovating at all is Apple, which has enough clout to ignore what the carriers tell them and just bring something to market that users might want. What can a startup do in this space? Sell their software platform to a phone company? Motorola is taking ten years to switch to Linux/Java platform. The only hope for mobile phones is that Wi-Fi or some other technology will become capable of competing with the carriers. Until then, most of the innovators will be holding their breath.
Getting anything interesting onto an existing carrier's phones is a nightmare. Not only will you need to make a better phone OS, you will have to start your own MVNO. Helio has made an attempt - their "ocean" device might be worth looking at.