Angel of Dimensions:
In addition to what you normally find around Phobos, every level has a "malign gateway" that spawns monsters. Destroying the gate gives xp but destroying spawns does not. AND stairs are unusable before the gate is closed. You don't have to kill everything but your job is to close the gate.
Dimensions:
Make it a special level (it could even reuse the name 'Hellgate'), the gate spawns one random monster per second till destroyed or closed (couple of levers for melee). Walking into the gate could warp you several floors or into a special challenge level (return to hells arena to defend your title?)
I started working on the special level described in these posts and it struck me, why don't we have a good old doomish door lever triggered ambush level?
It could even chain into that revisit to hell's arena. (is there any reason we can't chain special levels?)
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4 3 2 1
The numbers are wall dropped by the matched lever (labeled underneath)
Phase devices fail to work at all on this level
The globe next to the player is a large health, not a super
I know it still needs a lot of work but it's a start.