Prehard Stuff (some will be a repeat possibly):
1. Slime Staff needs to be Craftable, it's obsurdly rare and only serves 1 purpose if gotten later, and even then it's out done by other methods. If no, then the Antlion charger/Turret staff for early game summon.
2. Like 1, early Game Turrets. You see Turrets come up in hardmode, so the Antlion (spitter) will be the first using a few fallen stars, antlion mandibles and maybe some Silver/Tungsten bars?
3. Craftable/Buyable Javelins after have gotten one. Powerful early game, so after defeating Queen Bee or Skeletron with Jav in inventory.
4. Fishing.. Reduce the possibility of getting X item 5 times in a row and increase likelyhood of getting missing piece. For example, I have Sextant and other accessory, but missing Fisher Guide, out of all the rewards the Fisher guide will have highest priority. Same goes with having Golden Bug net but not yet gotten Hotline Fishing Rod.
5. Gladiator Armor dropped by the skeletons in Marble Mini biome get increased Throwing damage/special effect, otherwise it's just vanity with defense.
6. New invasion event for prehard. Hardmode has Pirates, Goblin Summoner added to Goblin Invasion, and Martians. Dunno what would be in it's place, but better then nothing at the time. Or even make it a All Day/Night long event, as long as it's prehardmode.
Hardmode:
1. Craftable traps and renewable Lihzard Bricks. Yeah yeah, there's a metric ton of bricks, but what if you wanted more but cant because you obliterated most if not all of the temple?
^Craftable Traps after Golem has been killed, Mechanic sells Piston Case blocks (decorative, material) and Goblin Tinker sells Auto Spears*, Mechanical Spiked Balls*, Jetflame Igniter*, and Dart Reloader*. Dart Reloader makes Normal Dart Trap found everywhere and then used with Lizhard material to make Super. Others just use Piston Cases+X Lizard Material+ Trap Type (5 or 10 so it's not extremely easy to make).
*All are Materials for Trap crafting and UNUSABLE otherwise.
2. Guide has Hardmode tips once a Demon/Crimson Altar has been smashed. IE: "I hear in the Underground of X-Evil and Hallow have creatures with souls worth hunting", with this a new player would be like "hey, I now know what to do first now that I am in hard mode".
3. A special Mixed Invasion, if Ninja's become a Invasion in some time, make a Special Spawner like the Solar Tablet where all that day you're fighting Ninjas AND Pirates.
4. A new Mob that causes "Ilusionary Distortion" debuff, like a Mirage, but what it does it makes a "shadow copy of yourself" fight you, but it's a distraction, takes damage but it wont hurt you. Causes Homing Projectiles to target it instead and makes you miss what you're trying to hit at minimal times.