The first season of Agent Carter was good, but it didn't set the ratings on fire. It looked like it might be a one-and-done series - even the creators were pushing on social media to have people contact ABC asking to bring the show back. Well, it seemed to work, as Agent Carter has been renewed for a second season.
Also coming back is Agents of SHIELD, somehow surviving into its third year. The show has improved a lot since its debut, when it was hamstrung by looming changes to the Marvel Cinematic Universe, but while it's better it's still hard for me to straight up recommend to people. I spent the night catching up on the last four episodes and it seems like the show has added a lot of sturm und drang, but not as much drama as it maybe should.
But that could all change in season three. The series has been slowly, torturously revealing The Inhumans, and now that they're out there - they even finally used that name for them in the most recent episode - perhaps season three can open up a little and figure out how to alternate between monster/Inhuman of the week stories and the kind of wheel-spinning longform storytelling they've been doing for the last month leading up to Age of Ultron. Also: they need to kill off a lot of the extraneous secondary characters who are cluttering up the series.
As for Agent Carter: I liked season one, but it felt to me like it needed to be better constructed. As a short series (and I do believe the vision for the series is to always be short runs) there's no excuse to have the show be as full of stops and starts as Agent Carter was. Put season two together like a miniseries and I think Agent Carter - which has a killer setting, strong writing and an incredible lead - could be the best Marvel TV series of all. Also: bring in some super characters! I'd love to see some of the Golden Age WWII heroes show up - Spirit of '76, Spitifire, Whizzer... or the original Baron von Strucker! There's a lot of fun to be had with these characters, and it would be super cool to see Peggy Carter leading some iteration of The Invaders as the young SHIELD's proto-Avengers.
Meanwhile the Agents of SHIELD spin-off, to have been called Mockingbird, got the axe before it really developed. That's sort of a bummer, as I quite like Adrienne Palicki as Bobbi Morse, but I guess this just means she'll stick around on the main show.