Questions about Half-Elves

[YungHarambe]

I am currently wondering if I could make a Half-Elf with a sub-race of Half-Dopple. I am pretty sure that my group is making the game home-brew but I dont want to stretch the reality of the game. (AKA break the game with a weird subrace)

 

[Twiggly the Gnome]

Are you married to the idea of the character being a half-elf/doppelganger, or do you just want a race that is an elf like shape changer? If it's the later, the firbolg in Volo's Guide to Monsters would seem to fit the bill.