I was bored and had many ideas so I have decided to compile them here. They are small challenges but also playstyles. These are just basic guidelines; feel free to add more if you wish (armor/ tool limits, etc). Begin!
1. Ballistics/ stuntman. Only weapons allowed are explosives, Molotov cocktails, and guns/rockets. This challenge will be tricky at the start but get easier as you go. Note that flare gun will count.
2. Elemental. Gem staves, blade of grass, waterbolt, starfury, and any other weapons you can think of that involve the elements of water, earth, air, nature, and the heavens. The world is literally the limit. Chlorophyte and the Duke's drops will be your best friend!
3. Ores, snores! No use of items that appear to be made of anything but metal. To make it harder, try lvl 2 of the challenge: every weapon you use has to be crafted.
4. Medivaelist. Bows, repeaters, swords, magic, spears, flails, guns like the boomstick and musket. No over-mechanisations!
5. Ninja. Throwing weapons, swords with a use time lower than 10-20 (adjust accordingly to how hard you want the challenge to be) and bows.
I will add more to my currently measly list but for now, enjoy! This is a sandbox game, so do wahteva ya want!