Here are a few cards that I saw really good ideas for, but the card was not made. Cut me some slack, I don't know how to do pixel art.
A card with a level up idea by Koey (http://boardgamegeek.com/user/Koey) on boardgamegeek. It might be a little powerful, I haven't actually tested it yet.
Original idea by Koey. A reference from Warcraft lore.
Original idea by Griffin (http://bossmonster.wikia.com/wiki/GriffinPatterson's_Custom_Cards). If you have enough damage in your dungeon but want more protection, this card is perfect!
Idea by TalixZero (http://boardgamegeek.com/user/TalixZero), but his original idea was to have 1 damage and deal it 3 times. That was insanely OP. I reduced it to .5 and deal it twice. This way, you can still do a lot of damage, but not 20.
Original idea by D0ct0rscarcr0w himself (http://bossmonster.wikia.com/wiki/D0ct0rscarecr0w%27s_Custom_Cards). I changed it from "to beginning of dungeon" to "back 2 rooms" and added on getting a spell if it is heads. It's also only once per turn.
Re-thought the card. I felt like it was a tiny bit underpowered. So I took the 1 damage away, but added the same effect that Minotaur's Maze has. But only made it possible to get 1 spell per turn.
Room idea and sprite made by Chesu (http://boardgamegeek.com/user/Chesu). A very powerful card to have at the end of your dungeon.
Original idea by Griffin. Added in only heroes in or at the entrance of a dungeon. This spell can be used to help you or hurt your foe. The choice is yours.
Original idea band sprite by Chesu. Changed it from 1 monster room and 1 damage to 2 monster rooms and 2 damage. Another spell that can help you or hurt your foe.
Idea by TalixZero (http://boardgamegeek.com/user/TalixZero), but his original idea was to have 1 damage and deal it 3 times. That was insanely OP. I reduced it to .5 and deal it twice. This way, you can still do a lot of damage, but not 20.
A room card that gets more powerful as you get more wounds. Can also be called Berserk Hall. Or Soul Room. Soul's Revenge. Could be a lot of names.
A miniature version of your Boss that levels with you.