Shadow Lairs are boring. They never change, but are just very difficult and take forever. So, why bother going there.

We could therefore use a fifth Arcade gate, a mission or another door at the terminal, which leads to Arcade, but with Shadow Lair monsters. Nobody cares what comes after. Apart from the view, Sanctum is lame and the HUD won't lock in the silly Passage.
It would be good to earn on average as much as the entry fee is, but this just determines how often people will play it, so it's not that important in the greater scheme of things.

//edit
It would also be nice to be able to change the loadout on most maps, like in most other arcade maps.