You are aware that people already make 'unrealistic' creations in these games already? People just use the gameface feature to do it along with the tattoo system.
But unless Ultimate Team is your main mode, you may well never even see it. Which brings me to my next point, what I'm proposing is an option, it doesn't have to affect everyone that plays the game.
For example; if you're a player that focuses on ranked, offline or doesn't particularly care for character creation.
Then it's very likely it would have no impact on your experience of the game.
Custom Attires isn't a new thing either, Fight Night Champion had 'Gladiator' styled trunks and Pantaloons for example. But you wouldn't have to deal with it unless you played Online World Championship (the equivalent of Ultimate Team at the time).
Now regarding the reebok deal, IMO it is holding back this aspect of the game. Even UFC 1 gave you a bunch of options for trunks and custom logos. Now everyone's kit is almost completely identical.