Actually I think the real reason to do Cathay and Kislev is that unlike the western European style lore nations they are harder proxy and I am pretty sure proxying was one of the main reasons WFB got its plug pulled last time and why Albion and Norsica are not being promoted.

Albion is just Celtic England, given the North sea being more dangerous then its real world counterpart setting wise it actually does make a degree of sense that they might still be low tech backwoods savages with chariots and the like compared to the Empire, etc. That said there are several historical mini options out there in plastic that would work as proxies for that since British Celts tend to be one the first opponent options plastic companies make if they are doing Romans despite them being in a rather unimportant backwater of the Empire not to mention the world of that day and age, and that’s not adding in tons of metal figures covering Celtic history and fantasy (frankly Albion was a rather bad marketing idea back in the 90’s due to over saturation making IPing it hard, from a pure marketing viewpoint they should sink it or have the Dark Elves, undead, etc. depopulate it completely when they do the new oldworld lore, actually undead Albion might be fun).

As to Norsica, as it stand there are 3 companies with historical plastic set proxy options at least, something like 2 to 3 more with fantasy options that would work in plastic, and that’s not adding the fact that Goths,Normans, etc. can work as them as well or the metals out there. Not to mention the Chaos Warrior miniatures already out there would cover them and they are kinda of just alternate list for that army. So yeah they are a lame idea even from a GW marketing perspective to promote, likewise this is Warhammer we are talking about why would we want “light and fluffy cultured fairy tail Vikings” for an epic  Grimdrep satire fantasy setting particularly since they where never really that way in the lore (this isn’t “How to Train Your Dragon the wargame”).

Russian and Chinese themed factions on the other hand make a lot of sense marketing wise (not a lot of competition in plastic and they can be fantasy-ed up in ways that will make IP lawyers and shareholders happy), as would an Arabian nights themed army and lore wise all those exist. While Cathay is not next to the Empire, they are next to the Chaos wastes and Ogres which if I recall right seemed to be the more competitive armies anyways.