A few Ideas I'd have liked to explore if I had time (and some money):

- Code/Product Documentation SaaS tool for early stage tech companies (very few good products, one is ReadMe.io)

- An ad network for progressive web apps (PWA have good future and as of now there is no exclusive ad network for them)

- CRM kind of SaaS app for startup founders (where founders can manage all their relationship, communication and contacts like investors, journalists, team, power users, etc.)

I wish you good luck.