OK is a logical choice when you look at the map. Kislev + Cathey doesn't really work without the Mountains of Mourn which lie in between them. You'd end up with a U shaped map with a massive gap in it.
And I don't know how they would do the combined map without the Chaos Dwarf wasteland. If they are not in at the start the might have to fill it with placeholders?
And they have plenty of room for new races to the south/east (Nippon, Ind) and 3 more Chaos Daemons.
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Nippon and Ind are on the table now that we have Cathay. CA has already left the realm of released WHFB armies.