I think I know what would demo 6's plot gonna be about.

I think demo 6 would take place in an underground city with anthro as its residents and no human characters other than Dr Sophie. Meaning that the wielder of the darkheart is not going to be a robloxian but an anthro character! Just come to think about it, the only anthro characters in block tales are: Terry, Jerry, and the Komodo Dragon. Of course there should be more lore about them in a future demo! Especially if we had lore about the undead, ghosts, and emotions in demo 3 so why not anthro?
Also we won't forget robots, machines, and mutants along with the anthro in demo 6!

But this is what I think demo 6 gonna be about so this is just my opinion. But let me know if you agree or disagree with this speculation (or theory) while saying accurate reasons but without harsh criticism.