I'd like to +1 increasing the respawn frequency (15 minutes maybe?), as well as making monsters build to a higher strength faster, so that people can get stronger spawns in dungeons that have been recently cleared. How much? Guess that's up to you. But I think that it's a problem.
As far as there being more dungeons recently, yeah, that's a great way to *partially* address it. But then, there are some dungeons that kinda lock out some classes, like Dvergheim with its discipline/tumble/parry checks being not great for casters, or places with lots of fear spells that aren't great for, say, rogues or fighters unless appropriately warded (which might be out of range of the available casters in the party -- not just a matter of preparing the wrong spells, but being unable).
Then you've got distance of travel and how long it takes to clear (Blaustein being a good distance to run from the outskirts, and Dvergeheim being rather long to clear, for instance), when people may be playing with limited time, as is often the case, and is probably why people try to visit the same few dungeons nearest the outskirts.