Partner with a finisher. Focus on your skills, collude at the start on the features, and let them take over and close the project up while you start up the next.
Finishing isn't really an intrinsic trait. It's a skill that most of us learn, often quite painfully. Why should those of us who've bothered to learn it have to compensate for the people who haven't?
You're right, everyone loves dreaming up new ideas. However, people who can put those dreams into motion are rare. I'd honestly hire a person who can start a project before putting it off onto some poor slob before I hire the poor slob. Why? Because poor slobs are incredibly easy to find. Starters aren't.
Ideally, your organization would consist only of people who can both start and finish projects. Something seems to get lost in translation when you have one group of people responsible for starting projects and another responsible for finishing them.
(Disclaimer: this may not apply to big companies. I can think of a few people within Google that I definitely would put into the "starter" category, and a few more that I would definitely put into the "finisher" category. Google is big enough to support that division of labor. Your startup is probably not.)