The game is balanced pretty well, which on the one hand is great because it means you tested it pretty well, and the game functions "as is" without anything being added. On the other hand it's hard to add new content, if one wanted to, without disrupting the game balance.
You have equippable items, it could be interesting to let the player find new equippable items in chests. Or swap an already equipped item. Finding a new item might completely change how the player interacts with the dungeon, making an impassable passage feasable. Or, one time use items, like an item that allows you to destroy a single bomb, or an item that reveals a single tile, or an item that reduces an non-dragon enemy damage/exp, etc... A common encounter in mine sweeper is the unavoidable 50/50 mine choice, the one-time use items could fix that.
Another Idea. You could also explore levels with different shapes, moving out from the traditional rectacle, making it look like a proper doungeon with halls and corridors and secret rooms (maybe behind cracked walls?). Hell, you can even make the level be so big the player has to scroll with its mouse to move from side to side.