As you would expect, initially the pickings were slim. I've been keeping an eye on these addons as they begin to take off. In fact, modding is so essential to the Elder Scrolls franchise that Curse announced it was opening its own site featuring Elder Scrolls Online addons, and ZAM launched its ESO addons website a few weeks ago.
Although we will likely not see the appearance modifications found in Morrowind, Oblivion, and Skyrim, we will see UI addons that help with grouping, dungeon crawling, and even roleplaying. The Elder Scrolls series is known for its mods and addons, and Elder Scrolls Online is no exception. Tamriel Infinium: Five essential Elder Scrolls Online addons
