I realize this kind of question is a bit of a gray area for the site, but following that disclaimer, I also know I can probably get a good answer here. The question is about reading order in the Ultimate universe, which had a limited scope and is hopefully therefore valid. Limiting it further, I'm restricting the question to ONLY trade paperbacks, i.e. collected editions, and primarily those covering the Avengers.
Following Ultimates 3 ("Who Killed the Scarlet Witch"), which book would be the next to cover the Ultimate Avengers storyline? It looks like Ultimatum would be the next major event, but after that the books seem to break up by character (ex. - Ultimates: Thor Reborn).
If the best answer constitutes a long list and is therefore invalid for this site, I will also accept a starting point (with hopefully a link for where to look following that).