1. a unistation, a combination of most (if not all) crafting stations for convenience. maybe it could require all the stations (forge, furnace, sawmill, etc.) maybe have different levels (adamantite, iron, etc.) and possibly some unique end game tools/armor sets that only the station could make? (slightly above turtle/beetle armor possibly with some unique buffs? like speed, or max summons, etc.)
2. Incredibly rare random vanity items from various mobs if you make any mobs (Ex. Flaming recon from halo 3 as rare as the slime staff (who doesn't like a flaming head, could be like the wisp in a bottle but your armor's head.) or an unusual hat from tf2, or even some king uniform, references to other games/media, etc. Vanity just to look cool and give you something to hunt for when you're bored.
3. Themed armor sets? possibly fill any gaps between progression with melee/mage/summoner/ranged sets?
4. more mounts? (No idea how people do custom mounts but hey you want ideas.) possibly a skeletron mount? or an eye of cthulhu mount, or even a brain mount.
5. more pets? (similar to mounts pretty much just random mobs as followers/pets that have a chance of dropping or are crafted by the mobs drops?)
6. New enemies, ranging from easy pre-hardmode to making you wish your end game gear was triple it's strength, Pretty much any animal, myth, etc you can think of that you'd like to see in the game, turn it into an appropriately fitting mob. Also end game expansions are always fun since with some gear things become too easy. (granted mods exist for that purpose, new mobs are always fun.)
and lastly, Decorative tiles. Trophies, banners, furniture, blocks even. Heck, mob statues maybe? You may not be able to get yourself an ocram port, but maybe turn him into a craftable and placable decorative tile? and maybe any mob that you feel like working on (vanilla or your mod) could drop itself as a decrotive tile. Trophy rooms can finally have floor based trophies. or maybe some animated tiles, like the living flame block except lightning, or anything like that. Maybe a tornado block with a little swirling twister, or a tornado in a jar animated block, or a mini tsunami in a jar that has a little beach flooded by a tsunami over and over. there are really a lot of options decoration wise.
I'm sorry if i've worded any of this badly, I'm really tired. So if you need some idea better explained, let me know. Although a majority of these ideas are terrible and very generic/basic.