They should just let the SR series go. I mean it's been going on since 2006 and has too many sequels in a short period. Just look at SR4, they released it pretty much instantly after SR3 because it was pretty much scrapped DLC ideas. If they really wish to continue the series, then they should just pick another name, sit back and think of a real story and start fresh.
They had to rebuild the engine between SR2 and 3 which why it took 3 so long to come around, when they had a functional engine they went for the $$ IMO both 3 and 4 lacking content.