I know such a thing is possible, but I really don't want to make a core mod to do it, the way I play and mod the sims pretty much depends on Nraas mod, a lot of it. Plus core mods are a lot of time and effort. Can you help me out? The reason why I want to make traits is so that I can make my own races and supernatural types, and make them unique, the ideas I have in my head should be possible, so long as I'm actually able to make traits. Or perhaps there is an easier way to add supernatural types to the game?

EDIT: also while we're at it, how would I upscale fairy wings and domes. I want to be able to build more under domes. And also would it be possible to make a slider that increases the size of the whole sim, which using slider limit mods could make giant sims.

And one other thing, how might I make it so that you can hire sims to perform skill tasks such as inventing, painting, alchemy etc.

I don't have first-hand experience on these topics so I'm afraid I can't offer much help.

Adding a custom trait is not that hard, but making it meaningful is another thing
and to make it interact with other core mechanics you'll probably need a core mod.
Look for the hypochondriac trait mod by Sims MX, you can find it HERE,
or to the custom traits present in the AwesomeMod.

About upscaling sims I guess you can use sliders, there is one for height too
(but I think that going too far with scaling will screw up a lot of existing animations).
OT: I remember Bethesda's suggestion for Skyrim about scaling: no more than 1.5x
and no less than .5x, in order to not deform too much the scaled objects.

To hire sims I suggest to look for douglasveiga's gardener service mod, HERE.