Hmm, let's see what we've got:
Lutins
Giants - Rare, but dangerous. And dumb, but not 'stupid', they can be fox sly.
Northern Humans - Make Lutins look charming. These are human clans akin to Vikings or Norse of yore, raiders and reavers when they go out from their isolated villages but soft-hearted caramel chews when they get back home.
Spies - These can come from pretty much anywhere. Nasojassa, Caralore, Lom, or southern kingdoms. They can come for various reasons as well, from the subtle gatherer of intelligence to the nefarious plotters of chaos. Have some care when dealing with the latter, but the former you can have plenty of fun with. They try to stay under the radar of Metamor, but occasionally their intelligence gathering raises the wrong eyebrow of curiosity.
Summoned Nasties - As mentioned above, one-off escaped summons or a small infestation of repeating summons that requires the destruction of the summoner or summoning location.
Giant Spiders - Ehhh, no? I have never seen them mentioned, and it would be far too cold for your conventional spider. Perhaps a magical summoning or some critter herded out of the frozen north for whatever reason. They've never been listed as native to the valley, so let's not introduce them at this late date.
Drunkards and Sadists - Just like any other society, Metamor has its unsavory side. Grayflank once touched upon this, but excessively so IMO. Played carefully you can have all the bad guys you could ever hope to deal with right at your doorstep.
So, as you can see, there is a never-ending source of conflict characters you can think up. So long as they're not disruptive to the kingdom as a whole they should be just fine (kingdom-affecting baddies tend to gain kingdom-weight response, and thus affect the threads of many existing plots). A thug holding sway over a small town, however, can easily surf under the notice of the Major Plots and still make for a very sterling piece of fiction.
Aaaaaand, then, there are the many, many other places where you can write of within the world of Metamor Keep that are not in that one tiny valley. A Metamor agent spying within Nasojassa, for example, or Lom perhaps? These kingdoms have never been touched other than by the overall brush of general political ethic and theme so the canvas is primed awaiting only detail paint.