It always feels really weird to me how rainforests are really desirable settling locations in Civ6, but have very few cities in real life. As I'm thinking about it, a big part is simply that a rainforest tile always has at least 2 food, whereas real rainforests are not particularly known for being rich in food resources (rather a lot of them have been cut down for farmland). One way that we might address this discrepancy is to just cut the +1 food bonus from having a rainforest. Since they only appear on plains tiles, that means all rainforest tiles will only have 1 food unimproved, which makes them much less attractive as a location to settle. To balance this out a bit for Civs with a rainforest bias like Brazil or Kongo, one idea might be to allow them (but only them) to get a +1 food bonus on rainforests.