If you've done all the core companion quests, which you pick up by talking to each character in their rooms, outside of Venesh/Sarrina and Claude which are forced for the main story, and have recruited the "optional" companion Esthera and done hers, then there should be two other NPC's in the world now. Sarcom is in the east garden area of Rockford to start that arc (Vena/Esthera quests completions needed) and Doanpra is in the gardens one screen above the college in Verdenhiem for that arc (NaN/Mira/Claude quests completions needed). Those are the other two main content releases that were put in after Tishna's.
There were a lot of smaller one off things there were scattered all along the way, such as the small demon group that you find during Claude's quest in that area of the world map and which unlocks something for another companion depending on choices that you can still go back and do and some stuff that was only available in act 1 like Zig. As long as you've done the main companion quests and explored around you should find most of the large stuff that's needed for the various companion endings.