What really bugs me about dresses is that we already have the flapper/hipster ones, so adding different, more contemporary ones wouldn't be any more complicated than items added to the game three friggin' years ago...
There's no additional issues with clipping, layering, etc., or walking animations (otherwise skirts would also be an issue - and we've had those in GTAO since launch, too). Literally zero reason why R* can't, so it's perplexing/frustrating that they still haven't.
That's why the ideal update theme would be something centred around nightlife, nightclubs, or even just 'stylish stuff' (maybe a Vinewood celebrity-ish thing) - again, there's NPC clothing in The Ballad of Gay Tony that's a perfect example of what R* could do:
Outside of GTA, other sandbox crime games have some great examples of what could be added - mentioned this aaages ago, but Sleeping Dogs has a few TBoGT-ish aspects, and does a similar good job when it comes to kitting out the more trendy, fashion-conscious NPCs:
(ignore the fact that Wei Shen's dressed as a shiny metal Shaolin warrior - most of the DLC bonus outfits are movie references, so ...yeah, just roll with it... )