Wow. There are a lot of really cool, and some scarily extensive ideas in this thread.
As someone new(ish) to the Sims (I had Sims Medieval on disc, and I played with the Sims 3 CAS way, way, way back when - and then I discovered Daz Studio for making character art.) I don't have any ideas of what I want this pack to be based on what's come before. I'm coming mostly from a combination of armchair Folklorist and TTRPG systems.
Personally, I'm expecting some sort of magic user - I'm going to use Mage - as the only life state. I hope they don't have an alternate appearance option, though. I like the idea of mages looking like everyone else.
There are too many types of magic to pick what I'm hoping for in magic's flavor. For the mechanics themselves, I'd like to see the magic system mirror the fame system, actually, rather than the vampire system, but with more branching. for example a choice of three powers at the beginning, and then as you get more Mage points, you can either pick a new level 1 power, or from a choice of three level 2 based on the first one you picked, and so forth.
I'd also like to see being a mage either be a choice of base CAS option (create a mage from the get go) or an Aspiration reward, kinda like the Strangerville storyline, but one that's intended for existing normal sims, rather than a newly created one (though that would still be an option, obviously. You create a normal sim and give them that aspiration.)
As for the pack's aesthetics ... I actually don't have much of an opinion, other than not Victorian gothic, because Vampires already has that covered. But I like variety in my mages, from Gandalf-esqe Wizards to Neo-Taoist Technoshamans and everything in between.
