Thieves Guild comes with a set of pretty useless passive skills? It's a no-buy for me. I remembered watching an old video from where devs were talking of Spell Crafting? What's up with that? There hasn't been any new Active skills released since the core game was released, no new classes, nothing! I doubt it will ever happen and I'm pretty certain some players would rage about the game being Play to Win, if they pack a set of new "usable" skills in a DLC. The core game needs to be updated with new content for sure.
Here's a suggestion.
Add some new quest lines, classes or some new skill lines to the core game, either in Cyrodiil, or Old Zones.
DLC's are nice if you want some more content to play, quests, group content, Pvp Areas, bosses & new item sets etc, but core game is being pretty much neglected.
Just my two cents.