Someone who could test different marketing channels and tell me which ones worked and which ones didn't. Would be more of a productized service. Once you found a good channel I would pay you a ongoing monthly fee to keep it maintained and re-test other channels.
Not having a decent mentor. Self taught working as a solo developer. Don't know what I don't know. If I had someone to peer review code or bounce idea's off then I could have progressed a lot faster. If I could start again I would join a development team and learn from developers with a lot more experience.