Nobody should really be complaining about 15 minute cooldowns to re-enter a dungeon. Unless you magically have every piece of DL or whatever from AV or CC you can finish one and port to the other to start it right away.

The real problem, that has been stated 100 times in this thread is HAVING to spam the dungeons at all. I see lots of decent ideas here on how to make changes.

My suggestion is to create a system that makes it easy to enter a specific dungeon once a day. For example farming the needed key items to let you in to said dungeon. On top of this (if a LS wanted to) you could hold multiple key items on different characters in your LS to be able to enter more than one time per day. Of course, this would mean drop rates would necessarily need to be raised OR loot sharing would need to be attached to these dungeons.