Nothing's been confirmed; at this point, all we have are rumors. A month or two ago I read that supposedly the (quite enjoyable) Age of Charlemagne DLC for Attila served as a sort of test-bed/precursor for Medieval III, but that's the extent of my "knowledge".
Personally, I won't believe Med3 is the next TW title in development unless/until it's officially announced by CA/Sega (although it would be very welcome news). Until then, I'm prepared for the next game (after Warhammer) to still be anything.
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