I can't see how the endings for Mass Effect 3 would be that hard to write around. One can assume the Reapers will be defeated, it's likely that it's only a matter of how it's done. Say the whole galaxy works together to defeat the Reapers with Shepard leading the charge and are successful with the Commander managing to survive. Mass Effect 4 couuld simply show the races getting along better, giving eachother a well deserved pat on the back for a job well done with Shepard being the living hero of the galaxy. You could hear the odd news repart of a school being named after him/her. Or him running for public office, whatever. OR, Shepard could die at the end of ME3 and we could hear about memorials being put up for him or her. Hell, they could name a damn planet after him/her. If the other races don't come screaming to the rescue in ME3 and Shepard beats them more or less out of sheer luck, then you could simply have alot more tension between whats left of humanity and the other races. And if by chance there is the option of the Reapers winning......well I guess what's the point of making a 4th game.