This is something I saw discussed on the S2 forums, and I have no way of knowing who had initially brought it up, but they deserve the actual credit for the idea.
I’d like to see an adjustment which would allow us to mix and match accessories between costumes for a particular character. This would allow a bit more variety for character customization - I, for example would love to use Thunder with his retro “wood axe” style tomahawks. This would not share accessories between characters, however, so giving Jago’s mace to Spinal isn’t an option.
Some of these might not work based on the physical bodies of any characters who might be different (Cinder and Riptor maybe?) but I think that if possible this would be an interesting option to have.
What do you think?
It would be interesting to say the least, and I’m happy to see you realize that it may not work under certain circumstances (which was going to initially be my 1st argument against you before I actually read your post), but realistically speaking, it’s probably just not going to happen. The reason is simple - it takes too much development time and effort for very little in return. So, in other words the devs likely won’t do it. If they do though, oh boy, that’ll make this a whole new ball game. 
This MAY be an issue, but I’m curious because it might not be as tough to do as it sounds.
Bear with me on this because I don’t know the first thing about making any of this stuff…
From what I’ve gathered on the forums over a long period of time, each fighter has a specific “rig” which seems to be a “stick figure” base that they build the rest of the model on top of. Some of the characters (in my opinion) seem to use the same rig for both costumes; let’s keep talking about Thunder since I look at him a lot. Both his standard costume and Retro look like they’re the same height and width, and his body looks like it has the same dimensions too. He also looks like he’s holding his weapons at the same angle, though obviously they’re of various shapes and sizes. Similarly, I think Orchid is also in the same boat.
I could be way off here, but it may be something that can be done (at least for some characters) without much work. Maybe one of the IG guys can shed some light on whether this is even something which might ever be possible to do without a lot of work. @TheKeits ?
The only problem with this is that some costumes have different accessory attachment points.
How do you work out retro Jago having weapon/boots/gloves when his basic outfit has weapon/mask/shins?
What pieces overwrite each other? Can you not have Jago’s masks equipped with his gloves?
What about potential clipping issues with accessories that are fit to one costume but not the other?
There are potentially a lot of complications, however this is something I’d like to see worked out.
I vaguely remember some dev saying that this is something they would like to see in a future update.
I find it interesting that the devs are willing to add or slightly change some things regarding the character models, such as adding jaws to Thunder’s bone headress or extra feathers on Riptor’s arms, but then they do a 180 and say that they can’t (or won’t) change some of the awful looking retros for the better (Orchid comes to mind) or other aspects of the models (Orchid’s hair, for example).