Just had the Worst/Best Idea ever. Ultron never dies, of course he has backups of backups and is constantly upgrading himself. So when A3 rolls around, I'd love it if Thanos comes by to destroy all life on Earth and Ultron is just like "Get in line, *****!".
Thanos would have to win, but at the end the Avengers might wonder if they are now allies with Ultron .... ?