The next DLC will give us the Thieves Guild, much as we know it from the single-player Elder Scrolls games.
There will be a new area to explore and a new skill line for any character which joines the thieves guild.
As usual with any major release there will also be updates to the base game alongside the new content. One of the more significant changes due this time is a further update to the grouping tool - finally we will be able to take cross-faction pre-formed groups into dungeons. Should make getting a group much easier and especially nice for truly cross-faction guilds which we, like many trading guilds, are.
Lots of discussion on our forums already as members try this out on PTS. Clearly it will particularly suit a khajit of our acquaintance :)