This is another idea for the Boss Lair competition. It would be the lair for a major monster, either an elemental or a dragon.
The basic idea is that this is a corner tile with a small maze that the heroes would have to traverse to reach the monster. It actually doesn't have to be in the corner, but I designed the room so the heroes should enter from the right or bottom edges.
Giving the monster a ranged attack is key, so that it can continue to guard the treasure in its nest. The gray tiles in the sketch are raised areas, more for interest than game effect.