For eons, our fellow hunters have been the only ones capable of doing pilgrimages into distant lands in order to acquire rare creatures.
So, since shamans are attuned to elements, why not do the same for them? If you go into the maelstrom and ‘do something’ to an earth elemental over there, why not change the model of your own spell? Same idea for frost mages, or even warlocks.
We have Earth, Nature, Fire and Frost (Frost mages) elementals this way (No air or water for enhance or resto yet). And demons are plentiful in their types, so I don’t know if, for example, it would be better to use the ‘voidlord’ category or just start going the hunter route and make them of that specialization instead for a more varied tank selection.
You could even implement it Soulshape style, with an npc in a capital city giving you a ton of options to change from your collection. (if you don’t want to marginalize inscriptors, make either the change cost a ‘generic glyph’ produced by them or to register a new one maybe?