The Night Mother is back,Rotten and Talking..They just assassinated an Emperor(alleged),They're recruiting...What could be the next plot for the Dark Brotherhood....Another Betrayal would repetitive and boring though and In addition to that What is Shadowmere?? Daedra,Aspect of Sithis,Fantasy Opposite of Shadowfax??

I would say just expanding the DB into other provinces, because by the time of SR, the DB were all but wiped out.