Overview
Having a rogue-like, dungeon-crawler element would be fun. The idea is to have mysterious ruins known as 'labyrinths' that adventurers can enter to seek treasure, rare items and fight powerful monsters.
Mechanics
Each level is increasingly complicated to navigate and increasingly treacherous, with deadlier and sneakier traps, and more powerful monsters.
Lore
The labyrinth is like a living thing. Monsters are drawn into it to settle and the very high mana levels cause them to evolve at accelerated rates. Monsters and people that die inside are quickly absorbed by it as if being eaten. Mana gets more powerful the more levels you progress, and so do the monsters.