We do need more food types and more ways. But the trees? They won't do anything and how will they grow?
The palm trees already grow in a tree crop plot. Perhaps there could be fruit trees? Oranges, apples, lemons... (I should note that citrus fruits are logical on the ocean, as vitamin C is needed to prevent scurvy. Yes, scurvy is a real disease and not just an insult thrown at low-ranking sailors.) Tropical fruits would be the main reason to have more trees. Other than that, I can't think of much else. Maybe some trees produce a sticky resin for water-proofing things? Resin could be useful for making boats.
I don't think the game needs more food at the moment, my entire playthrough I never had to worry about food. I could survive very easily off coconuts and shark meat with the occasional fish. Shark meat restores a lot of hunger and you get a lot of it. Water would be more deserving of improvements than food at the moment.
oh but maybe some fruits could have healing effects? or cure poisoning, if that mechanics is added?
Really, the coconut palm is passable because it has shallow roots and is salt tolerant. But citrus trees on a raft? I don't think that would wash. Scurvy only sets in after months at sea, and I really don't play this game for that long. Besides, you could get vitamin c from growing bell peppers You don't need trees.