I think the place needs an empire, but a titular one, not a de jure one. Poland-Hungary-Lithuania aren't a geographical unit like Britannia or Hispania is, they're just the leftovers that don't fit into the HRE, ERE or Russia. Even though imperial titles are important for anyone trying to unite lots of crowns, I don't think the three kingdoms have enough of a connection to each other to justify flinging de jure claims all over the place (although I think the same could be said for making Wallachia de jure ERE, etc).
I remember one mod automatically creates titular empires for just about every kingdom out there, and the more I think about this idea the more I like it. So in the absence of a de jure empire that makes sense, you can always found a Serbian or Hungarian or Lithuanian Empire.
And, of course, adding in your own custom titles is just a matter of changing some text files. You should be able to teach yourself how to do it in an afternoon.
I remember one mod automatically creates titular empires for just about every kingdom out there, and the more I think about this idea the more I like it. So in the absence of a de jure empire that makes sense, you can always found a Serbian or Hungarian or Lithuanian Empire.
And, of course, adding in your own custom titles is just a matter of changing some text files. You should be able to teach yourself how to do it in an afternoon.