I mean lets look at the obvious things..
1. It will not be a new EU, HOI, Imperator or Stellaris as all of the games are still being heavily worked on and there is still tons of $$ to be made from the current games.
2. Crusader Kings III is a possibility considering the age of the game however I find the aspect if CKIII being their new release unlikely even if they are finished releasing DLC's and/or updates to the game mainly because they as a company would want to leave a small gap in between CKII being finished and CKIII being released to ensure profit.
3. Victoria III is a possibility and I would say It is more than possible. The game is almost a decade old and has not received a new update or DLC in well forever meaning it is ripe for a new title.
A few people have stated this is impossible because Paradox already denied it however they could simply be teasing considering how Interactive PDX is with their fans.
Others have stated they wont make the game due to the niche time period of Victoria which meant it had trouble in sales however with PDX controlling a massive market of GSG games Victoria III would sell much much better than Vicky II.
Overall I think Vicky III is a decent contender even if it might have a different name.
I personally believe it is going to be a Cold War Era game my reasoning ?
1. PDX Con is being held in East Germany in a former 1950's Radio Broadcast station. Now there could be many other reasons to this maybe for more space, it was cheap to rent etc. But I believe its history with the Cold War shouldn't be overlooked.
2. It is one of the few era's that have not already been used that would not compete with other already released games. Any game that takes place before Imperator would without a doubt cause issues, a game between Imperator and CK2 would see the new title clashing to convince people to buy it as it would be going up against Paradox's most polished game (in my opinion) (CK2 not I:R).
3. The theme of PDX con just seems super Cold War like in my opinion.