I was thinking about that too. And i would like the option to play a goblin.
But why would you have to rewrite the game for that? If you are a goblin from a tribe that has no affiliation to the goblins the we encounter, you could do it just like any other race. Maybe there is a tribe thats more or less friendly, living near BG and even doing some trading in the city. Its a fantasy world, you just
need a story to make it work.
I think the Githyanki are problematic too. Usually i they are considered evil. And also behave like that. Going full Githyanki mode would make the story imposssible to finish. but what do we have? A gith that grew up in BG? Egg was lost somewhere and you were raised as a baldurian citizen. I think a goblin would be easier

.