If Justice League had to end, I'm glad it was with that episode. Except perhaps for last seasons four-part finale, that might be the best episode of any of the series ever in the Animated DCU.
Besides that, we know Superman's coming back in a direct-to-DVD movie, and some sort of movies for the Teen Titans have already been confirmed. I'd be shocked if JLU didn't warrant the same treatment in the near future.
Although, here's my ideal scenario:
WB and UPN just merged into the CW network, which decides to open up a children's programming block. Heading this block would be The New Teen Titans and Justice League Unlimited. To kick it all off: a three-part movie crossover event. Slade and the other major Titans villains join the Society. After JL villains wipe the floor with the Titans, the Society goes after the League. A few of the Unlimited escape the surprise attack and regroup with the Titans, then stage their own assault. The movie would explore the various hero-sidekick relationships between the teams, and would serve to give the Titans a new member or two (can we say Stargirl?).
And then both shows would air until the end of time. Hey, a guy can hope, right?