I want guavas! They always get left out of the tropical fruits club (I'm still sad they weren't included in New Leaf). I'd also like to see grapes, strawberries, and watermelons.
I want melon patches, now that we can have food farms/gardens. Honey dew is my favorite fruit irl, and the pumpkins are so cute in game, I'd love to see different kinds of melons in a similar vein to the multiple colors of pumpkins. (Also I hope the next game brings back perfect fruit...)
I would love for all kinds of berries to be added to the game! I think it would be fun if we each got one tree fruit and one bush fruit to start out with. Bushes have been in the game for a while I don't see why we can't have bush fruits.
I also really miss lemons and would like limes to go with them. Plums also sounds like an amazing idea.