MechaTarrasque
Hero
If every plane had a specific being (or type of being) associated with it, I would be okay with moving the planes stuff to the MM (since more outsiders would make me happy), but as far as I can tell Arcadia, Bytopia, Arborea (depending on whether eladrin are default fey or celestials), Ysgard, Acheron, and either Hades or Gehenna (depending whether where yugoloths come from vs. where most of them live) are left out. Now for most of these, I could see some kind of special NPC classes that you could stick on the fiendish or celestial (as appropriate) versions of orcs, hobgoblins, elves, dwarves, gnomes, and anyone who ends up in Ysgard. There could be Acheron eternal siege warriors; Ysgardian einherjar or maenads (always felt like the belonged better in wild Ysgard); or branali and firre's from Arborea.
The NPC idea also lets a DM more easily move away from species dependency (if that is your thing), so you could have a celestial orc branali or a fiendish halfling eternal siege warrior.
The NPC idea also lets a DM more easily move away from species dependency (if that is your thing), so you could have a celestial orc branali or a fiendish halfling eternal siege warrior.