Set after Rome II. http://www.bit-tech.net/news/gaming/2014/09/26/creative-assembly-announces-total-war-atill/1
I believe they have the rights to fantasy Warhammer, not 40K. Also, if this is still rocking the same engine then it is likely to be as big a mess as Rome 2, at least for launch.
They should just abandon this new crap that started with Napoleon and go back to Rome (not Rome 2!) / Medieval 2 engine and fix its shortcomings.
I thought Empire seemed pretty polished by the time I got around to playing it. Napoleon seemed to be worse to me, and from playing ROME2 in the free weekend on steam, the graphics looked a little worse than Empire to me. AI seemed a bit better, when it wasn't camping the capture point that is. The ships seemed to be pretty well bugged out in ROME2, twice during the prologue I had ships beach but then were unable to unload the troops, meaning they got massacred by ranged enemy troops.