according to reddit user tormentedragon, yoshida has yet to unveil a new harder version of current content w/e that means. but after that I know nothing else, which is why no new ultimate for 4.5 is problematic and since as you say there's the possibility of an expac release in late 2019 I'm worried just as you.
I mean we have many hints that they should make a 4.6 patch.
The fact that we should close the 3 lords questline in 3 trials and the palace at the center of eureka, but it seems yoshida squashed those hopes on E3
