Keys next and then combat and bat stealing and moving stuff, items (randomly placed), bridge for going through walls and the microdot to unlock the secret easter egg. I'm thinking I will use something of the mechanic of the Fyrdracas (or whatever they're called) in my Dig Dug game for the Dragon attack in Adventure:
Instead of the original's mouth-open mouth-closed cycle, my dragons will occasionally breath fire at the character. You'll have to get your attacks in between those attacks. The Bat will simply fly in, and unless you can exit the screen quickly enough, it will hit you and fly off with any item you have and drop it somewhere randomly in the dungeon.
I think I will not have levels in my version. Instead, all items and monsters will start in random locations. Except maybe the key to the yellow castle, which will start somewhere close to the yellow castle. My sound and music will be pretty primitive, unless I can convince my son Charlie to send some help from his his home at Dalhousie university.