One of my favorite ever adventures while playing in Rullaea involved a dragon's lair (and a dragon) in northwestern Severness. Another was an ancient Merethicles tower inhabited by an equally ancient alchemist and his everliving cronies.
I think, David, that "ploppin down a D&D style campaign in the middle of Eslin" is absotluely, totally appropriate! When we put together the descriptions of the realms (especially the organizations), our deliberate focus was on the webbing of the environment that the characters were going to be living and growing in - the cultural, if you will.
Also, we had a certain number of pages to work with, so we decided to create a solid foundation for GM's to build their own personalized stories, conflicts, ruins and dungeons upon (underneath?).
Once you have the surroundings and your version of them, placing the kind of ruins or sort of beasties becomes easier.
Also, we did take good a deal of care to try to weave in an element of interpretability into the monsters/dungeons/ruins bit of each setting, so that it can be different things to different people. The myths and legends section of Rullaea has some tasty possible tidbits to riff on, and some portable geographical references.
At a glance:
- secret hermitage and ghost guarding the relics of huinthall
- wise and strange beast in the hills north of Camleses
- the beast of morrowdown
- the shadow halls of erebus
- the black cubes
- the tomb of gel of melthisia
- ferenthrane the dragon
The stuff is there, it's just interpretable ideas, as opposed to hard facts and locations.
With regard to Monsters in the Woods, there are plenty of frontier and border spaces that you could exploit and people with beasties to waylay travelers. Eslin is Big and Roomy. I think that'd be a fun riff off of the basic template, and I'd feel proud that you tweaked it that way. Again, I don't think it's possible to be "incorrect" with any of our stuff.
It's your game now, use it in a way that you think best!