Kamen Rider: Heisei Generations Forever
 

"An era is ending- and everything is beginning..."

If you look closely at that silhouetted Rider (assuming it's ZI-O obviously), W and Den-O seem to be enlarged (aside from Build of course) within it. What could this possibly mean?

That the camera was zoomed in on them in their picture.
Same as Agito is off kilter in his.

I guess they're still using the Heisei Generations Moniker because Zi-O i the last Rider to debut in Heisei.

I wonder what the next period of Japan would be called?

I don't think the public is informed until the day the era begins.