I think Brian would be just as attracted to Justin if he were equally brilliant, but I do think it would probably have to be in some non-stigmatized field.... for instance, NOT being a fashion designer or hairdresser, even though those things are creative. For the same reason, I suspect if Justin were a brilliant dancer, it wouldn't work for Brian either.
I think he needs someone whose brilliance either DOES involve his intellect, or seems to... hence, painting, music, photography, sculpture, some forms of design... these things would all probably work, plus others I may not be thinking of.
But if Justin were a brilliant gardener and landscape designer... I can't see that. Brian likes things edgy, not just because he "likes" them but because I believe he values them more than non-edgy.
Something like architecture would work, but the problem is... how would you know you were a genius at architecture at 17 or 18? In an AU maybe.
I also think it's probably meaningful but not essential that Justin be good at something Brian isn't... having Justin be a businessman would seriously not work. I think it has to be something Brian can be a little in awe of, and not compete with.
As to me... I think it's just because Justin IS an artist, and that's who I formed my intense reaction to, and making him not an artist changes him beyond what I can "feel" as Justin. I just think being an artist is an intrinsic part of him, and you can't change something intrinsic about a character without changing them into someone else -- that's what "intrinsic" means!