The 3 W's of Business:
What - is the problem you are solving?
Who - is the customer that needs this?
Where - are they so you can market to them?
That's it.
If you're the CEO of a fast-growing startup, the mistake you'll make in the next year that you'll regret most will probably be a hiring mistake. Possibly hiring someone established, which you'll feel you should do now you're getting big, and which VCs will strongly encourage.