Survivor Mode
You fight boss's in a series and the get stronger and stronger. Team up with max of 6 people instead of 4. no vials or vitalpods. Just your equipment and your skill. Get as far as you can collecting the loot from the bosses along the way. This provides a more challenging game mode considering after a while boss's that were once hard become increasingly easier as you get better equipment.
More Customization
I have noticed that there is almost 0 customization in this game. Besides the color of the armor if you change it, and the color of your eyes from character creation, basically every one looks like everyone else when wearing the same stuff. Maybe the ability to make a costume on character creation that goes over your armor that changes nothing, but is just for looks. This way we don't have a ton of clones running around.
Any level joining
I guess this is not a total big deal considering you can just spam add 250 people and join people when they get to the floor you want, assuming they have room. Maybe make an item you can buy like the elevator pass that lets you choose what floor to join. This could be beneficial for three rings and the players considering if you got to buy it, people who spend money on this game will definitely get it. Why join depth 8 and fight your way to depth 17 for boss when you can join depth 14 and kill boss from there without wasting a ton of time getting there.
Pretty much all I got but here's an idea I would love but pretty sure 99% chance it will "never" get done.
Make a revamped completely better looking version of the game. Beautiful graphics, stunning details, awesome customization to create individuality. I know i am not the only one who would play it, if it was the same game but way better looking. Right now I feel like i am playing zelda which I do not mind, the game is fun. But how about zelda mixed with "Vindictus" graphics or some other high end hack and slash graphics.
Dude you can get armor and equip it as your costume! And the survivor mode sounds really cool!