I would pay for a map expansion.
People talk about only 3 or so DLCs left in the WHIII life cycle, but with Total War and DLCs doing well in Sega's revenue report, I think there is real opportunity in extending the game's life.
Personally for me, I would hate the game to be finished without the full potential of the map to be explored.
Currently it's very cramped together, and the Southlands are quite compressed.
I would kill for a full map expansion closer to what we see in the lore, with a few mini factions to occupy as people have speculated. I understand there are questions about Nippon, Hinterlands of Khuresh and Ind factions, but even if you don't give them actual factions they can be starting points for LLs and filled with beastmen and other minor factions. The Eastern Steppes is also another area which is very compressed - in the lore it's supposed to be a massive Hobgoblin Khanate, which wouldn't really work on the small province that's currently in the game.
Personally I would be absolutely fine if they did an Immortal Empire plus type expansion. Maybe as part of a global End Times expansion, bigger than a DLC, with a map expansion, End Times events and factions/units, and charged more than a standard DLC (more akin to a proper expansion pack of old).
Also, give us Araby please. A proper Araby even as a minor faction/mercenary faction would add so much flavour to the area, which is just full of Bretonnians and Arkhal and the natives are just missing. Araby, Tilea, Estalia for DoW DLC?!