If you could make impovements to Deep dungeon...

[Felis]

If you are successful get a guaranteed reward. If you are not, then no reward.
What's the problem with that?
Oh I know, it wouldn't artificially hamper player's time and effort as a cover for shallow and poorly presented content.

But that will already come (guranteed upgrade if defeated floor 50 boss)

Increase time limit from 60 to 90 (or more)

and reduce the respawn rate to balance out the exp/time again

Really, 30 extra minutes would be too much and would kill the challenge in solo mode. 5 or 10 extra minutes will be enough.