No matter what you choose, build stuff and be around smart people.
It's become popular in recent years to say that the idea doesn't matter.
Be suspicious of any work that is not building product or getting customers. It's easy to get sucked into an infrastructure rewrite death spiral.
The best people know that they should join a rocketship.
You want an idea that not many other people are working on, and it's okay if it doesn't sound big at first.
What you want to do is innovate on your product and your business model, management structure is not where I would try and innovate.