Startups are very hard no matter what you do; you may as well go after a big opportunity.
The best people know that they should join a rocketship.
In YC's case, the number one cause of early death for startups is cofounder blowups.
So you should always stay on top of people's vesting schedules.
Before product/market fit, your only job that matters is to build a great product.
You want an idea that not many other people are working on, and it's okay if it doesn't sound big at first.