I think the next update should be focused more on combat (like adding new weapons, armour, and bringing back the sword block mechanic) while also adding new structures, like castles, and more abandoned structures in the overworld.
There's no way to determine, we can only make an array of various guesses or suggestions.
In terms of combat, it seems to be worked on in the background for years, but nothing yet major has been revealed as an upcoming, solid release. New weapons like bladed or blunt handheld ones could be fun, if not also updated archery, provided any new weaponry get unique uses with balanced pros and cons and offering a greater variety of fighting styles. Sword-blocking is questionable, since shields exist in place of that, but if it were to be re-implemented, it should change to only being able to block melee attacks, while shields can block like explosions and ranged attacks.
Parity certainly appear to have been in the works during other major updates, so that's been gradually increasing yet still to see the full result.
Of course, considering the recent dimensional updates, it would be nice to see the End follow suit. But it shouldn't be a total overhaul of evidently different blocks as though emulating the overworld biomes -- it's supposed to be a barren dimension, so changes should be more subtle but effective. Like new biomes could consist of a chorus forest (more populated areas of the chorus plants), few new flora that are alien-looking and wispy (maybe a leafless, thorny bush, or a bulbous style of plant with teleportation abilities affecting any entities that come in contact with it), a more shattered-looking end island biome, and a biome with cave-like hollows in the island structures. There could be ruins dotted around, made of some end stone bricks.
If any revamps possible, update the end cities with a bit of extra challenges and a rare chance of a "basement" trapped loot room, and the main end island dragon fight. Even potentially to change the texture of the chorus flower.
If not yet the End, then another overworld biome update (there are few several that could use extra mobs, plant life, mechanics, overall uses).