Worlds of Alfheim
by RipvanwormerRelated to my thoughts about Goblin Park, here's a list of all the other realms (I think) accessible through Alfheim's magic points.
Plane of the Beastmen: The wizard Illodius summoned an army of beastmen from this plane in 550 AC. It's probably an outer plane, but we're not told anything else about it. Maybe the portal leads to the Hollow World or prehistoric Blackmoor.
Goblin world (goblin park): The goblins were fleeing an overwhelming enemy on an alternate universe 300 years ago. The goblins' foe draws closer and seeks to break into the world of Mystara. It could be the same as the plane of the Beastmen, but it doesn't have to be.
Dreamland (the Seelie Court) - as detailed in Tall Tales of the Wee Folk.
Source of the monsters of Thornbush: The portal leads to the "Sphere of Entropy" [Thorne?] and then to the Broken Lands. Beneath the Broken Lands, the shadowelves have more portals leading to other places monsters can be found. Most are probably native to Mystara, but they don't have to be. There are probably some creatures from the Sphere of Entropy that leak through into Alfheim. The World of Thorne from M5 Talons of Night shouldn't be overlooked, if only for the coincidence of names, but also because Thorne is linked to the Sphere of Entropy. It could be that the portal leads to Thorne, and then to the Broken Lands, and then - well, who knows. Wherever the shadowelves want the portals to go, I guess.
Source of the monsters of Dragontree: I don't really know where the dragons (and other creatures) are coming from here. They could be coming from elsewhere in Mystara, or conceivably even Veydra, where the gem dragons and Flaems were once imprisoned. Or some other world that's full of dragons. There are also quite a few incorporeal undead here too, so a connection to the Sphere of Entropy on the way to the dragon world isn't unlikely.