If it takes more than a sentence to explain what you are doing, it's almost always a sign that what you are doing is too complicated.
The company just needs to see you as like this maniacal execution machine.
If it works out, you're going to be working on this for 10 years.
If you want something in a deal, just ask for it.
In general, it's best if you're building something that you yourself need.
Startups are very hard no matter what you do; you may as well go after a big opportunity.