Samurai Champloo. Not that it didn't have a great ending, but I'd love to see more of it considering how short the series was and the fact that the end opened it up for that. Probably will never happen, but it was among my favorite animated shows of all-time. Avatar was one the best animated shows of all-time, and the ending was great. However the movie was horrible and made the series look like shit(they actually made fun of it in one of the last episodes). They have plenty of things to work with, especially from the comics on Aang's group.
As far as non-animated shows there aren't a lot out there, as most non-animated shows suffer from greedy networks overextending the show and the writing slowly getting worse over time as the plot starts to go nowhere(Dexter, X-Files, etc.). Those that are possible would be hard to do with the actors aging or even dying. Wire is an obvious one. There are multiple good shows that got canceled quickly that could be done, though the lack of popularity and the passing of time with the cast makes it hard to imagine it ever being done.
I personally think Game of Thrones is the best candidate of the current shows. It has a big enough universe that a movie could be great instead of a disservice to an already overextended show. Not with the current plot IMO as that should be wrapped up in the show, but I'd love to see them do a series of movies on Robert's Rebellion if they were able to get a good director with GRRM on the writing team. There is a shit ton of content there, really almost as much as the plot line they have followed in the show. Could make an awesome movie and would expand on characters that didn't get expanded as much due to early deaths; as well as showing many of the events that created the state show in the show and illuminating the details that have been compiled in to theories by the fans more so than they have already.