Board Thread:Mod Discussion/@comment-24489208-20131106230206/@comment-5665183-20140227113026

As someone who developed addons for WoW & Rift, I would add that it's highly unlikely to be possible to make your character appear different. Addons can generally only affect the UI layer that gets painted on top of the 3D game world: they can't reach into the 3D engine to modify the geometry, shading or itemization in any way. (WoW had an API that allowed this for a while, but they removed it because it trivialized PvE.)

In a nutshell, you can modify and add 2D elements like health bars, hot bars, etc. Addons are just there to make the UI more convenient or attractive. You can't affect loot, weapons, items or health in any way. The closest they can really come to new mechanics is tracking and displaying information. E.g., it might allow you to take notes on various places you've visited, or remember the location of nodes you gathered. They might also track statistics like how quickly you're earning money, how often you die, how well different items sell at different prices in an auction house, etc. Information tracking is where they are most useful. MMOs with no addons almost always have a poor economy for low-level items, because only with addons can you make it manageable to deal with the thousands of items floating around. GW2 is the only exception I've encountered, because of its clever market-rate system and relatively sparse itemization.

Someone mentioned a survival mod, and it'll make a good example. I haven't seen ESO's APIs yet, but I can use WoW's and Rift's as a blueprint for what's likely in an MMO. You could do a kind of survival mod that would track your location and force you to sit down for a while when you were getting cold, or eat some food when you were hungry. It could paint meters on the screen showing your coldness and hunger. It could even pop open a "you've died" dialog if you got too cold or hungry, possibly causing you to die when you clicked "OK" (if the game has some kind of suicide command like /stuck).

What it couldn't do is add cloaks, show you using a tent, reduce your damage if you're hungry or cold, reduce your health, change your appearance or otherwise modify any core game mechanics. In other words, you could make a sort of survival mod, but not one that anybody would want to use.