Not everyone should be able to use magic. It should be limited by some "talent". Not everyone can paint a portrait, let alone paint like Rembrandt. Not everyone can comprehend complex physics, let alone publish the general theory of relativity.
Whether this talent is a hard coded thing (such as intelligence restrictions) or just a game design allowance (all player characters have the talent, chosen NPCs have the talent) is kind of a toss-up. It depends on how you perceive or manifest magic in the game world: as an arcane art, an arcane science or a bit of both. As the system currently stands it is mostly limited by inteligence: high level spells will never be able to be cast by characters limited in the respective school of training and intelligence (to overcome base magicka costs), let alone cast those spells effectively.
As for splitting conjuration, I'm on the fence about it. But I really do find it ridiculous that the undead are conjured rather than raised. How hard is it to make an animation of a skeleton, ghost or zombie crawling (bursting?) from the ground while the actual geometric avatar is activated behind that illusion? It's pretty much what is already done with the conjuration cloud.
I'd like to carry an editable spellbook though. Something that I can customize what spells I carry. It would be nice if this was an actual in-game object with weight and a maximum capacity of spells. This would add to the usefulness of home bases: storage for spells, should the mage not want to carry his complete spell library (due to the weight and his/her - typically - low strength).
While I'm on the topic of books: I'd like to see editable books and editable maps. There's so much to do in this game that I'd like to be able to write a note to myself somehow within the game. Often the reason I abandon characters is because I haven't played in a few weeks and I can't remember what I was doing (due to the complexity of some objectives). For example: to enchant a flame blade with a character (who has minimal magic skills) I need to go get a soul gem, get a suitable blade, find or buy a soul trap scroll, hunt down a suitable creature for its soul (or go create a black soul gem - which means
two soul trap scrolls), gain entry to the arcane university and finally enchant the blade (everyone starts with a fire spell). If I save in the middle of this process I may forget that this was something I was doing and sell the soul gem. While this may seem contrived, it isn't (some people don't want to buy spells they'll never use or use spells just because they're there - there are many paths to fulfilling the same goal - that's why we love these games). Because the quest log doesn't track this sort of stuff, it would be nice to use the ability of modern consoles to record text data in the save for these purposes, maybe in the form of an editable book, scroll or parchment that can be carried.
I really miss the ability of editing maps (was it Daggerfall or Morrowind? was it even TES? it's been so long) with a label for a place to check out; or the type of enemy encountered; or that the location has a certain type of weapon, potion, etc. that can't be easily bought elsewhere.
As for all undead being human, I'm finding a lot of undead in Ayleid ruins. Including quest specific elven liches and zombies - think: Miscarcand. And I'm pretty sure there are a lot of elven ghosts (if not most). Dark elves summon ancestor ghosts... I don't know what else to say about that.