I believe that Minecraft could have a new sky dungeon in the overworld. This dungeon could be reached with blocks or an elytra, as it isn't fully intended to be an end-game dungeon. I don't think Minecraft needs more dimensions right now, I just think it needs more dungeons in the overworld to make the overworld have much more depth to it rather than just the focus being on getting to the stronghold to beat the game. Mineshafts are nice, but get stale after a while with the same thing over and over again. Ocean monuments are interesting, but again, they get a little bit stale after a while. (I forgot to include that jungle and desert temples also are okay, but they aren't extremely large or anything. They are pretty neat to find, and sure there are traps and explosions but it's all predictable and very easy to avoid. I think that in this dungeon, they could have traps and randomize them a bit to make it a little more tricky to get by.)
The dungeon should have a decent size to make it last a while instead of being a small area without much of a presence. The sky feels quite empty (Except for phantoms, which only spawn in certain conditions) and could be a little more interesting. It should be more rare than both an ocean monument and mineshaft, though with a lower rarity than a stronghold. I believe there could be new mobs that are located within the dungeon, or they could just be normal mobs/mob variants depending on what the dungeon is like. It could have a boss, or it could just have a lot of exploration/puzzles included inside. The loot could range from somewhat good to extremely valuable items, along with the loot possibly being scattered about or could be in one location. Maybe an entirely new item for the game could be received? The dungeon could have a name perhaps like the Sky Temple or the Flying Islands, but those are just some simple suggestions. (Sorry I say "dungeon" so much, it's just that I have nothing else in particular to call it.) The dungeon could have some new blocks included, just like how the ocean monument has it's own blocks that can only be found at the dungeon. This is about all I really have to offer in terms of suggesting things, I'm glad you spent the time to read this and have a good day/night.