Dragonball's already gone about as far as they can take it as a series. Granted, I'm very curious about Dragonball Online, but I can't see them making a continuation manga/anime work, simply due to the way "Z" jumped the shark. While Goku is a kid, he fights strong people. Then he fights armies...then a 'demon' who was probably the strongest person on the planet. Things are all well and good at this point. Then Raditz shows up and is a bit stronger than Goku...then Nappa and Vegeta show up and at least one of those two were strong enough to destroy Earth. At this point the audience is going "O_O". Then we get to the namek arc which tells us that Vegeta was at the upper end of small-fry in a professional planet-exterminator's private army. We're just about hitting the series' high-point about now, despite the coining of the fan expression "Are they still on Namek?" giving voice to the sheer arc fatigue it was suffering a the end.
Then the androids come...then Cell, who came very close to destroying earth...and then we get Majinn Buu whose arc consisted almost entirely of curb-stomp battles one way or another, and who was enough of a threat to the universe as a whole to warrant the undivided attention of the Supreme Kai. Creating a credible threat after that point is like trying to make an arc after Tengan Toppen Gurren Laggan's anti-spiral arc (The OVA version, no less). They'd already pushed the protagonists power-levels into the realm of gods (in fact, the 'gods' of the Dragonball universe were very strongly implied to be MUCH weaker than the saiyan cast members by the end of the series), the series as a whole is basically the poster child for the concept of "Power Creep". It had a fun run, to be sure, but it's better off ending with Z than trying to continue the series-proper.