They're not really build-based, but here are some very weird and random suggestions I came up with on the spot:
- Dwarf Mode: You may only attack using picks and explosives (and rockets in the late-game, I guess, also including explosive bullets and anything >80% explosive). Also, as soon as you can (i.e. once you have wood, sand and iron/lead), craft lots of ale and maintain the Tipsy (de)buff throughout the whole game.
- You must throw your pick away and mine everything with explosives only. (Mix and match with the one above.)
- Boss Rush: All boss battles must pit you against at least 2 bosses simultaneously. In hardmode, only hardmode bosses qualify. (So for example, you must fight the Lunatic Cultist with a mechanical boss (hardmode), not the Eye of Cthulhu (pre-hardmode).)
- (Early-game only challenge) You are not allowed to harvest any wood (any variant, including cactus and pumpkin, and including the wood of underground cabins) until you have eaten a Life Crystal.
- Buckethead: As soon as you acquire the materials, craft a bucket and you must wear it in the head slot (the ACTUAL armor slot, NOT vanity!) for the entire playthrough. (This also prevents you from getting any armor set bonus.
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- All 3 of your armor pieces must be from different armor sets.
- NPCs on Holiday: You may only house NPCs in the ocean biome, and you MUST occupy BOTH oceans (optional: try to split them as evenly as you can). This makes accessing them very inconvenient which could change your playstyle, and possibly makes invasion defense interesting.
- Kingdom in the Sky: Set your spawn point in the house of a floating island (lend him wings to reach it, and give him a bed) and never remove it. (The loot is yours to take.) Gift the island with a rope leading down to the surface. You are not allowed to place any furniture (excepting platforms and campfires) below Space altitude.
- Every time a boss fight ends (whether in victory or failure), you must trash the last weapon you used against it. (You can re-acquire that weapon by grinding for it again if you want.) This includes summon weapons ("last" being the last time you waved the weapon, not the last time the minion/sentry hit the boss).
- Play on Expert Mode. You must keep all your money on your person at all times, i.e. you cannot deposit it anywhere. (Let's see how rich you get by the time you defeat the Moon Lord. >
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- (This is really evil) You are not allowed to remove your starting 3 copper tools, as well as any stone, dirt, mud or mudstone (and their respective walls) from your inventory via trashing, dropping, selling, placing or depositing. (Mark those items as "Favourite".) You can remove the stone via crafting, but the dirt and derivatives will slowly clutter up your inventory. Bonus: Make the last blow on every boss with the copper shortsword. (You'd probably want to use a flask for that.)
Have fun.