I think the only point in making a Batman Beyond franchise would be to build towards the "Return of the Joker" story, which (let's be honest) is the most epic storyline that came out of Batman Beyond. The Joker's return is something that equally difficult for both Terry AND Bruce. It presents them both with real challenges, and allows for something close to that incredible flashback sequence.
So IMO, a Batman Beyond series would have to follow an established Batman film series. There would have to be some continuity there, like there was between B:TAS and Batman Beyond. The audience should have a general idea of the history of Bruce Wayne/Batman/Gotham, as well as side characters within that universe. It'd make the viewing experience so much better for us die-hard fans, as well as casual fans/viewers.
Now, TDK trilogy is over. I think we need to see a new Bruce Wayne-Batman series which will be tied in with this potential DCU that WB is trying to build. THEN, after 3 or 4 movies, I think Batman Beyond would be the next logical step.