I use the concept of matrix and matrices quite often when describing my campaign milieu. Basically it means the rules I've set up for how reality works in my ... errr ... reality. Elves can't be arch-mages, humans can't simultaneously be 2 different classes at once, player-characters can't see in the dark, and so on ad infinitum and ad nauseum. For instance, I don't have cross-breed races IMC such as grizzled half-orc veteran or effete (this word can be typed with one hand) half-elf spell-slinger.
So what do I do when I get some dude or dudette who really wants to play a variant character? Simple! I allow them to enter the milieu from outside the matrix. I make the prospective player come up with a justification for the variant PC being there. The might include an interdimensional gateway, cursed scroll, very angry deity, magical accident, and so on. If they spin a good enough tale? I allow it.
This requires a bit of adjudication on my part and a dialog with both the incoming player (and variant character) and the existing group. In this way I can keep the established boundaries of my game world while still allowing the players some freedom.