there is no vicious reward item suited for manausers! So maybe
a magic amulet:
+1 manareg per tile
Faster mana regeneration is dangerous since it tends to break Binlor and Mystera. Also, you need to be careful with attack penalties since caster-Assassins can ignore them rather easily. The Mage Plate is already too much like this.
get full mana (usable once per level)
Crystal Ball already works very similarly... except the crystal ball needs more work to charge up and
has an activation cost. This would be obscenely overpowered, and much better than the crystal ball used to be when it was overshadowing every other item in the game.