We have a Steam Page for Dominions 6Most notably, we're getting an entirely new magical path: Glamour. It's picking up 'illusions, phantasms, dreams and the stuff of forgotten legends' from Air magic, so we'll have to see how its inclusion works on the battlefield and what Air gets in compensation. Scales also seem to be more dynamic, with your choice of nation and pretender affecting their limits. Riders and mounts are also going to be separate entities, though I'm not sure how much that'll affect battles. Speaking of battles, unit sizes seem to be tweaked as well to be more granular, though I don't see the point that change. The only reason I can think of it's to make hoburgs no longer as small as bugs.
Obviously we're getting new nations as well, and one is even shown off in the store images. Pyrène is a nation obviously based on the Basques, that has gone from dwelling in caves to blood sacrifices to goat-demons, so Marignon is going to have a fun time with them. Pyrène also gets sacred Mouflon cavalry, which is awesome.