What kind of spells do you like?
Elemental POWER
What kind of effects do you want?
As much as the developers can dream up and implement. Elemental temporary weapons, sharp projectiles, the traditional balls, shields, impair effects (instead of damage), uses that don't relate to attack or defense...
Do you like flashy spells that look pretty?
Sure, as long as not every spell is flashy. Some extremely powerful spells should be extemely understated, while others should telegraph AWESOME and deliver excessively.
Do you like to summon hordes of imps?
No. I don't like imps. A small squad of Dremora or atronachs is more my style.
No. Magicka is fine by me!
Do you want magic to affect the world around you?
If magic has special interactions with the world, no. If the magic system has effects that aren't available by other means, fine. But if fire will burn a particular plant, I want torches, enchanted swords, maces with a Fire Damage salve applied, and fireballs all to do the job. So basically, I want non-magic sources of an effect to work the same as magic, and the only "magic restriction" is where an effect is only available through magic.
I actually have a catalogue of a few hundred "magics" that I'd find fun to play with, but several conflict with Tamrielic lore. Unless Bethesda would like to add the ability to create mildly sentient beings to the game without the use of souls, anyway.