Hey Whitebear,
I totally agree with you on that. People wont be creating maps on their own, especially because of the XNA stuff Magicka uses.
However, I don't think that should stop modders.
Magicka actually allows modding by changing .xml files, and there's a really wide range of things you can do with them.
The only thing you are basically limited to, are the maps you play on.
So just to get things clear: You take an existing (piece of a) map and add or remove anything to it you want. NPC's, enemies, drops like weapons or magicks.
You can even have npc's kill eachother or give them different dialogs.
Basically you could say: You have as much power as those guys which create the DLC's. Except for the part that you cannot introduce new NPC's/enemies/weapons/magicks/maps.
So if you want, you can create a whole new campaign (Note: as a challenge map, hence the reason the new DLC's are challenge maps).
So although I agree on you that creating new models and maps etc for Magicka will probably not be done, modding in Magicka is very much possible. And in a way, pretty powerful too!
That basically leaves us again with one question, why isn't there a Steam Workshop for Magicka yet?
Your fellow fireball catcher,
Dikharz