I've done a ton of enterprise web development in the past 8 years and I would like to develop some new open source project that people would happy to use and run it as an online service.
Don't let the few responses/upvotes discourage you. The question comes up almost every week (similarly "what service would you pay $10 or more per month for?"). I think it's better to listen in on non-technical forums or FB groups. E.g. dentists, logistics companies, construction, researchers. A friend recently started an app for pharmacies.