I made collaborative painting apps, https://hellopaint.io and https://malmal.io (there might be some slight NSFW content). In the best months I made 800€+ in ad revenue from malmal but currently it's a lot less. I think there's potential to make a lot more though, although I'd like to stop showing ads and switch to some more predictable income model. I do have a patreon but it only brings in ~100€ per month. I could promote it more though.