Here are some ideas my friends and me made up:
-A map and/or compass
-Houses on island(maybe the compass can show you the way back home)
-Villages
-NPC Quests & trading/shopping
-Being able to build multiple Rafts
-Bigger sails, that make you go faster
-Animals that can be found on islands like pigs, cows etc.
-An animal farm on your raft
Let me know what you think of these! :)