Back when I was still running a node, I was making 10-12% APY at my peak. I was playing the LOOP liquidity game quite aggressively. I doubt you can still make as much as market inefficiencies tend to go down when more people are doing it.
Selling liquidity? Check what LNBig charges. Check how much deezy.io offers for opening a channel with them, and check how much they charge for one. Check https://lightning.engineering/pool/ and their rates. Etc.
Check how much content creators get streamed on fountain.fm and see if you have something to offer.
As for the limitless number of ideas, check out Kevin Rooke's podcast on how many people are experimenting to integrate LN in everyday businesses.
I have not been in the routing/liquidity game for a while, so can't give you hard numbers. But I don't remember many nodes being profitable. Most people just do it to support the network and because it is fun. That's also the reason I started. I got carried away, becoming a top 50 node for a while, but this was not reasonable using Umbrel on a rPi.
Hard numbers on which one?
I have not been in the routing/liquidity game for a while, so can't give you hard numbers. But I don't remember many nodes being profitable. Most people just do it to support the network and because it is fun. That's also the reason I started. I got carried away, becoming a top 50 node for a while, but this was not reasonable using Umbrel on a rPi.