I always wanted to have a battle inside of a burning castle or something like that, probably an Escape map. Every turn, the fire would spread, which, in addition to burning your units if you stand in the flames, would damage the structure, such that rooms and paths collapse, encouraging a faster pace. Eventually, escape will be impossible. There could be NPCs to rescue or chests to open too, which you would lose out on if you took too long to travel through the map.
I also think it'd be kinda cool if you didn't meet certain requirements in a previous chapter, the castle would already be partially damaged by the fire from the beginning, making it tougher.