“An anime you wish never ended and continued on”
Not too long ago, it was announced that Naruto is ending soon; that’s not my answer, it’s just a good reminder that even the longest series eventually end. Of course, some series are better suited to have indefinite lengths than others.
So I’m going to split my answer here.
Series that could have been any length: Cowboy Bebop. Most episodes were stand-alone anyway. And even if someone wanted to revive it after it ended, the way the series ended doesn’t have to be a barrier; there’s already a Cowboy Bebop movie that was made after the series but set before the last few episodes.
Series that ended with obvious loose ends or sequel hooks: considering the way that it did end, The Big O. The ending it did get was even more confusing than the TV ending for Evangelion, and it never got continued or remade in any form.
Speaking of remakes, I still haven’t seen it yet (waiting for the official subtitle release next year), but I hear Shin Mazinger Z Impact ends with one of the biggest unresolved cliffhangers in anime, setting up for what presumably would have been a new version of Great Mazinger that was never made.