I'm 100 percent certain that this has been suggested before, but I figured I'd give it a go regardless.
When I say soldier customization, I mean something akin to the XCOM 2012 system, where you mix and match things like hair and hair color, face, skin color, armor color, helmets, etc.
The majority of the work required to implement this would consist of drawing tons and tons of new sprites, something that I'm not entirely sure is feasible (and it's also modifying a lot of original game art assets). But I think all of us at one point have wanted to give our soldiers jungle camo on jungle terrain missions or to give face masks to soldiers on terror missions to conceal their identities.
I realize that this feature and purely cosmetic and that currently there are other priorities in mind for the OpenXcom team, but at some point or another after 1.0 I wound like to see this happen.