Opportunity to earn a cut on every transaction. How big should my cut be?
jonnyw: Be careful.  You should either be part of this company or get paid a set fee as a developer.  Any other solution isn't stable.  A company can't really survive with someone else owning its core IP (Option B), nor would they want to keep paying a perpetual cut for a product that you'll presumably complete/deliver at some point.  If you want to go down Option C, take an equity stake.