Well, I would want something that you could run when you had just aftermarket wheels and it wouldn't look like you had a foot extra of fenders on each side. Also I think it would be neat if you were able to run a 3.5 LT kit on it and not have to move up to larger fenders for legal coverage.
Body lines should match up, I think some of the glass companies do it nicely but others dont. The headlight should be integrated nicely, I dont really like how some companies have space between the headlight and the fender.
Also Im just throwing this out there. I like what ford has done on the raptor, it keeps with the body lines of the F150, and it has the factory style fender flares on it.
I know I cant post a picture of it without this thread spirling down, but notice the flared fenders, but still with the black flares on it and it looks factory. I would like to see this but maybe a primered flare that you could color match.