In Season 11, Phil had a list of 15 teams he thought should be invited back. 10 teams joined, and Eric & Danielle were the 11th team. The other five teams were Colin & Christie (who accepted but withdrew because Christie got pregnant), Dave & Lori (S9), Kris & Jon, Flo & Drew (S3 hybrid team), and Kelly & Jon (S4).
In S18, all I know is that Brook & Claire wanted to join but withdrew because Claire got pregnant. Also, I read on TVtropes that Steve & Linda (S14) were emergency alternates and was at the starting line city (in case something happens to one of the teams).
In S24, there was a leak of 10 teams that would join, names included Tim & Marie, Pam & Winnie, and Andy & Tommy (you know, the teams that are more ALL STAR than who we got). I'm guessing they didn't expect so much of their initial picks to not want to return. But then again, I don't know if that list is even official.