Guildleves are pretty "Content light." Easy for the player to get in get out. Most can be done in a 10 min period.

How would you feel if they introduced guildleves that lasted the full 30 min? I imagine they would work something like this:

Your leve targets spawn and upon defeat, another wave spawns fairly close by, and again and gain, Every 5th Spawn a mini-NM spawns. You are rewarded at the end of the 30 min based on how many "waves" you've defeated.

These leves would be more oriented to party play, and replace the "camp and grind" type mechanics.
Maybe offer 2-3 of these each day make them non-linkable so if you get a group of 4 members you have at least 4 hours of consumable content.

If you lack the time to complete 2 or 3 - 30 min leves, you can still pick up the standard leves which can be done solo in 10-15 min sessions.

I personally would love to see something like this implemented. I believe it'd foster a nice group play community and be fairly easy and inexpensive to implement on the development side.