I would like to see support for limited texture mods. That is, the ability to use, say, a third party hi-res pack. From my understanding, the primary objection to these in MMOs is when people abuse the system, for example making enemies neon lime colored so they can be more easily spotted. But the legitimate applications for this (character customization, higher res mods, etc) should have no impact on the server, and would make TES:O one of the most customizable MMOs on the market, which could easily add some value by way of contrast.
As for preventing abuse, I'd say the first step would be to constrain the alpha channel as appropriate, on a per-replaced-texture basis, to prevent see-thru exploits. The bright color mods hardly add more advantage than turning up gamma levels, and given that they detract from the game experience more than they add, perhaps this would be best left alone.
Aside from those items, I feel it would only add to the quality of the game. That said, I would *not* suggest tying this directly to the addon scripts, as hotswapping textures would allow for trivialization.