SK could probably use an 'infinite dungeon' mode, where you can just go down and down and down forever and the enemies get harder and harder with no cap on their stats, just a linear increase until you decide you absolutely can't take it anymore no matter how good your gear is
I assume random flavors of clockwork tunnels would be easiest to use for this
hmmm...quite interesting, i might say...and it could be opened by going at least one time to the core. there could be middle stops where you can actually change your equips, from time to time (like, 10 floors), and heal points every 10th or 15th floor. Also, to make this interesting, you can make a group in the first floor, but no one can't enter the group after the beggining (it locks itslef automatically). And, to finish it up, there could be a lessened energy cost per floor, depending on the depth (let's say, reduce 1 energy per floor after the depth 20, then 2 after 30 (emberlight dificulty), then 3 after 40 (after-core dificulty), and so on, caping at 5 energy floor (where you would pratically be using most of your energy in revives)).