I'm working on a party and one of my players wants to be a tiefling with an air genasi mother and a tiefling father. They want to Look like a tiefling but have the race bonuses of an air genasi.
In addition to this, this player wants to have 'wings' that are purely for the aesthetic but move when they cast Levitate. This is understandable and I think it's a great concept, with the addition to other additional information they cobbled together to make the reasoning behind their unique circumstances (which will take way too long to list) that I think will fit in with my campaign...but a large majority of the other party members are now wanting to do something akin to the tiefling/genasi player.
It goes from simple stuff like a human who's eyes glow red because they have some ancestor that had some orc blood in them to some wild explanations like "my mother's father's wife was a Dragonborn and my dad's great great grandfather was a rock gnome so I want to be able to spit fire AND have the ability to tinker tiny contraptions!" (yes, this is an actual example from a player)
I think the tiefling/genasi is alright and letting someone add rock gnome's tinkering abilities to a character who's race and class doesn't qualify for it seems harmless but I have no idea how destructive it might be in the endgame.
The question is: To what extent should I allow my players to modify their races' stats? I don't want to have too many confusing stat changes and I don't want to seclude everyone from concept ideas that they like but I'm willing to cut it if it's unacceptable. Any advice is highly appreciated.
Note: None of them are sorcerers and they're starting at level 2 Edit: The issue I'm more conflicted with are the players who want to mix and match race mechanics.