I think it would be useful to be able to change skins in-game while playing minecraft. I'll get to the uses after I explain how it can work.
First, this works almost the same way a texture pack works, there is a folder specifically for skins, you can compress the .jpg form of the skin into a .zip easily, so then you move it to that specific folder. Next you go to the options while in game and there's a section called 'Change Skin'. Click it and it has the same layout as the texture pack menu. It shows a preview of the skin, and the name of the file.
Before you say 'if it was possible, texture packs would be the same way', just think, would changing texture packs mid-game be useful at all? Also when in mulitplayer, changing your skin would generate MUCH LESS lag than changing packs.
As for the uses, this is what I had in mind. Say you have a forest camouflage skin an you're being hunted down by enemy soldiers, and there's a massive desert in front of you. You can't just leave the game! You need to quickly hide, change skins to a desert suit, and run as fast as you can across that desert. Also if you're showing skins to a buddy it would be much more convinient.
TL;DR: Armies need this.
Please leave feedback, as I'm not sure that this would 100% work, I'm just spouting this out from a pure assumption that it could work.
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You could craft a wardrobe item, and when you right click on it, you can browse the installed skins.
If someone changes to a skin you don't have, you automatically download it to your game, sort of the way Sprays work in source games, or how custom textures work in TrackMania. Since the skin images are so small, it'd literally take just a few seconds, during which only the client downloading the skin would be (if at all) lagging.
I'm interested to see rank-based skins, so on faction servers, all the soldiers, generals, kings, mercants, miners, etc, have rank/job based skin for each team. This could make team battles more interesting.