What's hard mode?
ok let me say how much has change the game since 1.0:
1. the corruption isnt the only spreading biome, there are other 2 and the jungle near end game.
2. reaching the underworld is only one of several things that happen in the first half of the game. the boss defeated there opens the other half of the game, which not only modifies the world but also makes it harder than before which means you basically have to start all over again in terms of progression (your high tier armor at that point becomes basically the equivalent of starting with wooden/copper).
3. the jungle got two new locations, not gonna spoil which ones
4. you can fish now, with an npc, gear and accesories. fish are used in potions, apart of that they dont do much.
5. several visual revamps and item changes
6. several quality of life changes to ease the gameplay like adding items to favourites, sorting inventories, dedicated slots for the grappling hook, etc..
7. there was a crossover event with Dungeon Defenders 2 which added an NPC and a tower defense event to the world, it also reinforced one of the "Classes" of the game.
8. the 1.3 updated added an "Expert" option when making a new world, Expert worlds changes several key mechanics in the game which means its harder to survive even the first night. but also increases the loot chance among with adding special reward bags to bosses which add unique stuff that you cannot get in a normal world.
9. thousands of items, equipment, blocks, mobs, etc..
10. new NPCs that give several different stuff.
11. new events and updates to old events like the Goblin Army or Blood Moons.
12. new building paths for stuff that was more or less limited to obtain.
i hope this summarizes the evolution of the game. honestly, just update it and start with a new char in an expert world. the game has changed a lot since then.