The maps are very full, but remember that you can do "facets" so you can have 12 different Lodors or 5 different Sosarias. Just saying in case you want to recycle dungeons to make different ones...although they will look like their replicates but it still provides a blank area to work with. One thing I was considering was to take the dungeons as a different facet and link "like" ones together to make a big dungeon. Like taking every fire type dungeon in the game and then making a big fire dungeon while setting it to something maybe higher than level 4 in difficulty. Put in some new fire creatures and such.
As for a Skara-type quest, maybe look at other 80's games and draw inspiration...like a quest that throws all of your gear in your bank box (with no access to your bank box) and throws you at the bottom of a dungeon (like Dungeon Master or Eye of the Beholder) and you have to get to the top to escape...while finding equipment along the way. Sure, you are a Titan of Ether or an Alien (if you have those) with alot of skills...but now you are rushing to find a club because you are naked and weaponless. Once you get to the top of the dungeon, you escape back into the land.
cool idea!
We saved quite a bit of real estate by redoing the underworld map which allowed us to add a number of different dungeons, but not enough for a full skara quest like you did.
But I have a problem with adding harder and harder dungeons/mobs. I found that players could quickly reach a point where nothing is challenging anymore. If you just added harder dungeons/mobs then that just gives them 1-2 weeks more of play before they reach that cap again.
That's why I added the diminishing return system for most of the stats/damage/etc. makes it increasingly difficult to max anything - at low level 1 point adds one point, then you need 2 points to add 1 point, then 3 points to add 1 point, etc.
In that vein, i'd add any new content with recycling existing content in mind... for example, evil badguy comes and curses you. You have a chance of 3 of 4 different curses - one curse reduces your damage/damage of pets, another reduces your health/str, another makes you slow, etc. this type of quest/content makes sure that the current content stays relevant. A player would then have to play the game in the current world to uncurse himself.
There's already a TON of content here. I don't think the answer is adding more... rather finding ways to make the content the player doesn't want to engage in more relevant.
in this example, a cursed-slow player would not be able to kill demons anymore, walking/hitting/casting slower. they would have to go back to normal dungeons for adventuring. the quest to become uncursed could require ingredients from lower end dungeons.