Making Race Traits independent of race

I’m preparing my new campaign for my group of players and I was wondering if a ruling would unbalance the table.

What will be the effects, balancing-wise, if I let players choose a race for the traits, and another race for the cosmetic appareance of their PCs ?

For example, they could have a PC that is human, but has all the traits of a gnome, in order to make a good wizard. I feel it would enhance RP, since they will be much more confortable with their character. Most of them are experienced players, while others are kinda new, so I unsured if it will unbalance them too much (since the experienced players like to min/max their characters).