I have a great idea for Terraria that would increase replayability exponentially.
Diablo-style items.
No, really, before you laugh, think about it. We have probably well over a hundred (two hundred?) items now, and we're able to just find a significant portion of them, and craft the rest. Even just for findable items, this would be amazing.
The idea goes like this: When finding (or optionally crafting) an equippable item (any item that can go in one of your equip slot, or possibly even just holdable items like how the water tube thing works), the item has a smallish chance to have some bonus(es), sort of like the "enchantments" in Diablo. For instance... Copper Helm of Pain. 10% damage-given-out increase while equipped. Gives a whole new reason to go back to wearing the item, doesn't it? At least until you find a gold helm with 15% damage increase and 10% longer breath when in water. So people would hunt even harder for chests and such, lest they contain an item with that perfect bonus. Or maybe you would just randomly craft an item that had a bonus. That would have people scrambling for every material they could squeeze out of the world, just to keep crafting.
Alternate idea, possibly in addition to the above: rarer (findable/craftable on a chance) equippable items with sockets. Every gemstone gives a certain sort of bonus, but is socketed permanently. The gemstones we have now would give only small bonuses, but perhaps there could be rarer, higher tiers of gemstones that gave larger bonuses, that would give people a bigger reason to keep mining them.
What do you guys think? I think there's not a chance in heck it would be added in Terraria, but it would certainly make the metagame and single player interesting, and keep the completionist addicted. After all, Terraria is based in a large part around the items.