Okay then probably anything with standard mission goals (three keys, reactor/boss robot, and exit) is likely okay then. Including your own stuff.
![Smile :)](./images/smilies/icon_smile.gif)
Although boss levels in The Lost Levels will be troublesome, but maybe there's a way to just skip those levels.
On the other hand I'm guessing Entropy Experiment 1/2, Konflict at Karon, EAF2, and (maybe?) Project Mandrill are likely stuff to avoid for coop. Shame because a lot of those missions are really big classics.
EDIT: Actually, why not try Obsidian? More players = more chance that questions of 'what did that switch do?' will be answered sooner.
![Very Happy :D](./images/smilies/icon_biggrin.gif)
You could try instructing different players to look around in different areas of the level, and have one report when they find a newly opened area.