Frank Welker has just confirmed that there will be a sequel to SCOOB! as we heard back in June. He answers this on his website (the 7th entry down) where he says "My next Scooby Doo project is working on the sequel to “Scoob” and we have just completed the first session. There will be many more recordings before we are finished."
In addition in a GalaxyCon interview, Frank reveals he will be in Velma. While it sounds like he won't be playing Fred or Scooby, he does say the show "is loosely based on the Scooby-Doo characters, and it's kind of a different version of all that. But the character I play is a little different sounding for me, so when I come back to it, they'll play a reference tape."
Thank you so much to Ben for sharing this info with me!