Belts should look better.

Ability to choose if your belt goes above or below your jacket has been heavily requested for a long time, however its a very tricky issue to handle.

You'd almost have to make multiple models for every single belt cosmetic to work with every unique Jacket. Some Jackets are very long, some are long and bulky, some are short with little wiggle room between the jacket and shirt. Some the Jacket is very open (meaning you'd see a fair bit of the belt length ways at the front) but some are very closed (meaning you'd barely see any of it).

There's already a Similar System with hair, where certain hats will cut off the top of the hair but still leave the back, or headbands that press on your hair such as compressing the afro around the headband. I really doubt they'd want to make another similar system, one that would be much harder.

This would mean you would have to make a couple of models for the belts to suit all jackets.
Above jacket
Under long-open Jacket
Under long-closed Jacket
Under short-open Jacket
Under short-closed Jacket
And even then there's bound to be some jackets that are special and there would still be issues with belts clipping through Jackets.

@sir-kringle with the cutback on cosmetics as of recent honestly I think remaking multiple variations for every current and future belt would be an entire seasons worth of content.