I read about this on wikipedia, so take this at a grain assult, but there is talk that in the next DTM Scooby Doo movie, the Pup Named Scooby Doo characters and time period would play a key role.
I don't know if the movie would just take place in the PNSD time period, and use those characters. Or will the grown up characters appear, and the PNSD time period would be used flashbacks, or even the grown up characters going back in time to their childhood. I would love to see the PNSD characters, meet their older selfs.
We might see Red Herring as a teenager or young adult (depending on the age of the post Zoombie Island era Scooby Doo gang).
But just a rumor, read about it here.