Hmm This is an important question to me with respect to making the world feel more complete even if it is limited as part of the allure of Terraria is exploring the world finding what is there in order to progress then going back to see how it changed later after progression influences major events across the world. Thus every area should have a reason to return there.
As of now the main major biome candidates are likely The Ocean and Hell additionally you could make an argument that some more subtle aspects of the corruption crimson and possibly hallow could be warranted to make the places feel more alive as they spread through the world. Particularly their underground portions) are likely the biggest cases of lackluster revisits.
The ocean really suffers from limited unique content outside of Duke Fisheron in late hardmode and several prehardmode enemies moreover if the hardmode biomes spread to it while it will still be able to spawn ocean mobs as of 1.3 they no longer spawn with the same frequency. Before Duke it mainly serves as the best place to find Water chests however this is not guaranteed and they can generate anywhere in the watery part of the world. Now I already posted a suggestion back in 1.2 along with some others.
Hell or the underworld is amazing prehardmode and a vital component of progression, however there isn't really any incentive to go back outside of two new enemies that only spawn post mech and drop some rather lackluster items. Lava Bats share the Hell bats drop pool and behavior and Red Devils weapon drop is... exceedingly lackluster compared to the magic weapons crafted from the mechs. The flame wings aren't bad but considering how ice wings are far easier to get and don't require you to go out of your way it doesn't have enough draw to the area.
The spreading biomes aesthetically don't seem to be on the same level as some others and are quite unequal in their enemies for instance the corruption, particularly the underground corruption, lacks any water enemies outside of normal cavern foes and corrupt goldfish which are a joke by that point of the game even on expert. in fact I feel that behavior wise The underground Corruption is probably the easiest of the hardmode underground biomes as of its unique enemies Corruptor's projectiles can be destroyed and they have predictable AI. World Feeder's are just larger worms with no special traits other than dropping cursed flames(Seriously they can't even inflict cursed inferno) and The slimes are well slimes. This leaves Clingers as the only major threat outside of the normal hardmode Cavern foes particularly when entering large caves where they are most effective.
The crimson underground is even more limited as blood feeders and blood jellies only spawn in water Leaving only Floaty Gross and Ichor Stickers along with the standard cavern foes as threats out of the water. And let us not forget Ichor Stickers suicidal tactics near lava it is almost more effective to just ignore them in the lava caverns if they can't reach you and come back to pick up the souls/ichor/keys later... (the only enemies here that don't suicide in lava are Crimson Axes and Floaty Gross)
As for hallow same deal with the lava layer and it doesn't even have any unique water mobs.The slime are quite simple behaviorally so the main unique foe is the Chaos elemental with its teleportation ability. Really all three of these rely primarily on the normal cavern mobs to be a threat to a player. It just feels thematically off for these cavern foes to be so dominate over the evil biome enemies in threat levels. (Giant Bats=Confusion, Armored Skeletons=Broken Armor, Skeleton Archer=range+DOT)
Sure their are the biome mimics but they only attack if attacked first and can be summoned by the player intentionally so...
As for the Cavern it could indeed use more mini biomes in particular I would love to see more attention towards the behavior of mosses as they are currently only aesthetic in nature and are mostly destroyed by the hardmode spreading biomes. They could at least be given the ability to be planted outside of the Staff of Regrowth. Considering the mechanics are in place that would be a very small fix say have the Dryad sell them perhaps at night? Also rather than destroying moss perhaps the evil biomes could convert it to corrupted/crimsoned/hallowed moss as well(perfect for growing pesky thorns or other features to liven up the caverns
)
Fire Moss Granite and Marble caves however certainly could use attention. Fire moss is non-renewable, pretty and destroyed by corruption. And Marble/Granite caves lack real incentives outside of building materials (Javelins and the Medusa head are nice but I can't see them alone as being worth it.
Considering the greek inspiration of the marble caves there are so many good possibilities to add enemies down there! And the granite biome is limited to only two drops(though the renewable granite is really really nice and probably my favorite building material but that isn't enough of an excuse) They should have at least one water enemy for prehardmode and hardmode respectively in my opinion...(and then at least 2-3 optional unique drop gear) As that feels about right in terms of visitation incentive...
