Hey Cloud9! Just was looking through my feed and saw this and remembered it so decided to see if anything new happened. This suggestion box hasn't gotten any love yet so lets see what I can come up with...
Most basic of basics: A super powerful secret item that takes multiple steps to unlock and is set according to your character. Can anyone say...Mokais Godsword? Kidding. Though if you do decide to add this it would best be done when the maps are finalized as you would need to get the lore set up for it and make it out to be something amazing. You can take this further by having a secret weapon in game for each possible character class with there own back story. There also end game items so I don't expect to see these for a long time.
Demon Army: I know the tower keeps the demons sealed from what I remember, but I was wondering if it was possible to corrupt your character or start off as a demon and lead them like a kingdom. It would be interesting to say the least.
Magic Spells: There are plenty of these you can have. Heck you can go the Final Fantasy route and have upgraded spells as you go along like Fire 1, 2, 3, etc. Add them in as books or something that teach you the skill or unlocks for quests when it comes to the master level ones. Heck, for some you can even add lore. Something like "Hero A created this spell because B and defeated army C and saved Nation D." Or something.
Hell Tower Levels: This is probably really self explanatory. The idea is that you are fighting the Demon Army so why not have levels in the tower that represent that? My idea was to have the tower stretch below floor 1 for this and have them accessible at a higher floor through stairs or an elevator as it would make sense to have hell be beneath the tower instead of above.
Angels, Divines and Religion Lore: Self explanatory really. Maybe add in some more lore for these kind of things as it lets you get creative and it flushes out the world a bit more and makes it feel more alive and real.
Classes: If I remember there isn't a Mage, Healer, Archer, or Monk class (though there are items for the last so maybe). It would be interesting to see what skills and items you can come up with for these.
These are drop of the hat ideas I came up with really fast and most are probably in the works. Either way it would still be interesting to hear back.