Ooh, I got plenty of ideas. For starts, a mechanic idea:

Configurable Estus/Cerulean Flasks. Having this option on disables ALL natural Health and Mana regeneration during boss fights or events, and disables all usage of health, mana, and buff potions. The flasks refill at a campfire, but only if there is no boss or event active, and also refill on death. Having this option on also halves mana usage during bosses and events.

Estus starts out healing 100 health per use, and Cerulean regenerating 80 mana per use. The player starts out being able to hold 3 flasks total. The player can configure how many of their flasks are Estus or Cerulean at a campfire.

Killing crucial/semi-crucial bosses to progression will upgrade the tier of the flasks by +1, increasing Estus healing by 30 and Cerulean healing by 15. Killing non-crucial bosses or clearing non-crucial events increases your total flasks by +1.

To balance buff potions, all buff potions will now cost between 5 and 40 mana to consume once a battle starts depending on how strong they are.

Would just be super fun to have no-cooldown no-sickness health and mana potions with the downside of them being extremely limited and scarce during tense fights. It would make Mage a bit harder to play solely based on the fact you're limiting the amount of overall damage they can do, but at the same time you could also add items that restore small amounts of mana on killing enemies (that includes statue enemies) to have players innovate and become creative with how they make their arenas and plan their fights.

But more important than that, a change I think should be MOST pertinent in the mod:

Make weapons/tools usable below their required stat lines, with the exception that they deal 1/10 of the damage they normally do. This completely gets rid of the issue many players will have with Pickaxes/Axes/Bombs and not being able to use them without the right builds, and allows players to focus more into magical or summoning classes if they so wish.