No matter what you choose, build stuff and be around smart people.
One thing that founders forget is that after they hire employees, they have to retain them.
These all sounded really bad, but they turned out to be good. If they had sounded really good, there would have been too many people working on them.
You need unstoppable people. You want people that are just going to get it done.
In YC experience, 2 or 3 co-founders seems to be about perfect.
Remember that the idea will expand, and become more ambitious as you go.