Yet another micropayments startup. Unfortunately, my current thoughts on that require several hundred thousand, minimum, so I'll need to get rich (relative to where I am) before I can really get that done.
I'm also not particularly interested in P2P payments, as there are lots and lots of systems out there for that, and it never has really taken off. The dominant system is still merchant/customer, and I think it will continue to be for a number of reasons: each person receiving payments needs a lot of people paying to make any given activity worth doing, and people just don't seem to pay each other tiny sums -- and paycheck-size sums are already handled well by check/ACH, direct deposit, and so on.
But given the level of funding I'll need to get this started properly, it certainly isn't a YC app, so I have to succeed with some other interesting project first. :)