Legends Z-A seems to be the most mysterious Pokemon game ever. The reveal trailer didnt really tell us anything beyond the basics (the title, region and very ambigious idea of plot) and its been half a year since then and we havent got a single update. Not to mention the idea of a whole Pokemon game to take place in one city is something they've never done before and even Legends Arcues doesnt feel as revolutionary as the premise of Z-A (afterall we already had a feel of open world in SWSH wild areas).

Also unrelated but do you think the concepts for PLZA could've already exist during USUM? Here's what Sina and Dexio say about Zygarde:
Dexio "Zygarde is said to maintain the order of the Kalos region...so then why is it in Alola? Does it have something to do with the Ultra Beasts that have appeared?"
Sina "Who knows...? Maybe Zygarde itself will tell us someday."