WH40K has very good plots. It doesn't have a well-developed metaplot, however. The only recent metaplot of note was the 13th Black Crusade, which has not concluded to my knowledge (and probably won't until the next edition of the game) and was really a way of rationalising a load of tournament games.
There are numerous individual storylines that are interesting in W40K (transmitted via computer games and novels), whilst of course GMs can adopt any tone they wish. However, the overall metaplot of the entire setting won't change too radically (despite the novelists - particularly Dan Abnett's - wishes) if doing so either makes the setting less fun to play in or shuts down certain options in the setting (removing entire races by killing them off would fall into that bracket).
Seeing how the Forgotten Realms was systematically wrecked by too many major paradigm-shifting events in succession, culminating in the moronic Spellplague, I am fine with the setting changing only in small ways and not major ones.