[Spoilers] There can totally be a Mass Effect 4 as an MMO, with a galactic civilization even with the endings as they are.

By now, everyone should know how easy it is for them to shit on your decisions and their own cannon: (TIM getting the reaper no matter what, Rachni being a thing no matter what, Udina being counselor no matter what, Shepard "Stopping the reaper threat!" even if by cannon, Arrival means the Sol system and any other with a relay should be toast)

Illos had a functional relay to the citadel - Illos is said to have had other people go there after the Geth attack on the citadel (which is why the Council say that Vigil simply stopped working in ME2), the reapers in the destroy ending fall over on Earth.

Both of these open up the possibility of Mass Relay technology (or at least some crazy freaking FTL technology like the Reaper ships used when the Citadel relay option was gone), to be recreated.

In fact, an MMO might consist of exactly that - rebuilding the galaxy, holding the new peace, exploring new technology and competing over old reaper technology.

And who knows, maybe some systems simply won't be visited in the next whenever because the Normandy's location during its flight simply wasn't known - and they can still have their little isolated Stargazer/Child shit going on well into the uncontacted future.

There'll be more ME content, at the very least, even if it has nothing to do with this - it'd still be stuff we pay for, right?