it would be fun, but not very helpful...well in game it would be helpful but in real life a giant slow tank would have quickly been surrounded by t34s and explodered.
A fun variation of it would be a secret project research that takes quite a while, and you don't actually know what it is until its done. basically it could be a nuke, it could be a v2, it could be a giant tank, or it could be something ridiculous and useless, basically making it a kamikaze type last ditch effort that could either turn the tide or fail miserably.
Maybe have a 1 percent chance of it being that nuke on cities of the underworld that Hitler made to tunnel into the core of the earth and explode, ending the world. Or a small chance that you open up a portal to another dimension and unleash a demon army on Earth. It would be a very fun mod, but as far as dlc's go, i wont count on it.
I hope they bring back the aliens from hoi2, only make them more detailed. Like all the countries at war can have an event like when Nationalist and Communist China declare a truce to defeat the invaders.