Instead we had either a soul value of 150/300/600/1200/1600 and that's it, wouldn't it be more fun for some of the really powerful individuals in Oblivion (Mannimarco, Mankar Camoran, Umaril, etc...) to have really big soul values like 2400/3200?
It would allow us to make some really powerful enchantments, yet it wouldn't be overpowering because you'd first have to successfully trap their soul, and since they're unique you can only do so once. With other enemies, you'd be able to repeat the process when they respawn and if you regret making an enchantment you could always trap a similar soul. But since there are only a few unique individuals with higher soul values, and they can only be trapped once, you'd really have to be sure what to enchant with them.
And why not allow us to have multiple effects on clothing and jewelry too? We could do so on weapons but not on armor, it's not like it would be more powerful or anything. You're still limited by a set soul value, so you'd have to choose between a single powerful effect or dilute it over multiple smaller effects.
EDIT: And please, make the different soul gems look a bit more different from each other like they did in Morrowind. In Oblivion they had slightly different size and colour but that's about it.