From the BotCon twitter...

It will NOT be a con set theme we have done before NOR will it be set in a Universe that we have used for a con set before.
So.... it shouldn't be a theme that they've done at a previous BotCon (like Beast Wars, Machine Wars, Mirror-verse, etc), nor will it be a story set in an existing Universe used for a previous BotCon (like Wings Universe, Gen1/BW Universe, Mirror Universe). However, it should be noted that the tweet was a clarification, that now suggests that they have used this theme somewhere in their past toys or Club Comic, but NOT as a previous BotCon comic or box-set of toys.

Unfortunately, they have 10 years and over 150 toys to process to narrow down what that could mean. It would probably be quicker just waiting the two weeks and seeing the reveal.
But does anyone have any guesses as to what that might include? (a theme used by the Club, but not used as a BotCon theme)