There are so many missions, so many wasted bosses lying there... waiting to be easily slaughtered with few hits by a well-geared merc during his casual farming route...
So why don't you harness what you already have? why not enhance the system, renew it?
Also, in this way you can even buy some time to publish some new well-crafted content...
I was thinking something simple:
1- At level cap you unlock the "dungeon progression level" system (DPL): starting from level 1 up to 10 or 30 or infinite, whatever.
2- You can repeat the dungeon just like as it is right now (5 times daily cap, 6 with VIP/VVIP) while being able to choose a DPL (Dungeon Progression Level) based on your progression so far (disable the DPL mode if you only want to farm that dungeon).
At day 1 you can do a specific dungeon starting from DPL1 ("Dungeon Progression Level 1"): if you succeed, you unlock DPL2, but you will be able to play it and progress further only the next day.
3- Each week or month (month is better imo) the levels RESET, so everything gets resetted down to DPL1 again.
4- A new dedicated seal, maybe called "DPL seals" with unique rewards
5- Weekly/monthly missions (e.g. "reach DPL10 with 3 dungeons", "complete a DPL10 with all guild members", and so on)
6- Obviously, each subsequent level will be harder than the previous one, there may even be different mobs or additional boss mechanics, such as one-shot mechanics, hardcore stuff (if Pestis grabs you, you can escape by smashing the "E" key on your keyboard up until DPL 10, but starting from DPL11 you're simply dead, just like Agares when he grabs you).
7- No Goddess Graces are allowed (just like in Niflheim)
8- Each level will have a small evil core and DPL seal dropping percentage increase (just like LUK), but each 5 DPL that percentage will be 100% (+1 additional core)
9- The higher the DPL, the greater the rewards: every 5 DPL you get +1 evil core and +1 DPL seal, +2 evil cores and DPL seals if within a group, +3 if within a guild group... In this way, you can still solo it, however, it will be highly recommeded to do it within a party due to the amount of rewards
10- Maybe, If you do it with all guild members, you also get guild seals and GP (just like the current guild dungeon) and a guild record history board within the guild house gets updated (along with the donation ranks).
I listed these points to help giving you an idea but the purpose is simply to bring all the dungeons we currently have back to life
, no matter how.
Otherwise they are just wasted content.