Thanks to Stephen Cribb, we have the name of what will probably be the next DTV. Awhile ago, rumors were circulated about a Scooby-Doo film about King Arthur, as it was posted on a LinkedIn page of a known WB executive.
A quick Google search of "Scooby-Doo and King Arthur" pulled up actor Greg Ellis's official website, in which he has an article saying he recently completed recording lines for a DTV called "Scooby-Doo in King Arthur's Court." Greg Ellis will be voicing the villain in this film, as he directly mentions saying the line“And, I would have gotten away with it too if it wasn’t for you meddling kids and that mangy mutt of yours!"
Thanks so much again to Stephen Cribb on the site for prompting me to Google the title and find Greg's page. Stay tuned for more info!