Some decal positioning tools similar to War Thunder would be cool. You can move, scale, rotate and double sided render up to 4 decals over your current skin. It allows for quite a lot of creativity in customization while reusing the same few hundred assets.
They also have skinning competitions and submissions where the popular ones get included in the game at a later update.
I like the idea of creating your own, but it's not practical. A detailed skin for a large ship would have multi layers and use up many MB per skin. Having to sent that through the server for every player so everyone can see each others custom decorations isn't practical. Especially when you factor in peoples poor taste and infringement of copyright laws.
But more customization in the livery department would be very nice indeed.