Doctor Moustache wrote...
Boiny Bunny wrote...
Doctor Moustache wrote...
I can see it being something like the Yuuzhan Vong of Star Wars EU happening. After the war with the Reapers there is a short age of peace and rebuilding but before the galaxy can really recuperate a brand new threat appears in the form of an invading species or even an empire of multiple species from beyond citadel space.
Sadly, that's the most likely plot of whatever will come next. I fear that such a plot would destroy the plot of the original trilogy however - as whenever this happens in popular media, the implication is that the original enemy from the first installment was a joke and the new one is much worse.
I don't want to be told that the Reapers and everything which happened in ME1 - 3 was a joke... 
Not really though. That would only be assuming the galaxy is still at full strength, which shortly after the reaper war I highly doubt that. Resources will be spread thin, species will of grown back into their old hatreds, and the invading force would be taking advantage of that. They wouldn't have to be stronger then the reapers, they would just have to be more tactical. Reapers are over-confident they have to strategy anymore they just roll in and invade like brutes. The new invaders could be more manipulative behind the scenes and **** us up from the inside then take over. Then the games could be about rebelling against them after they've already won. Okay now its sounding even more like Star Wars, but hey its the primary influence of Mass Effect for a reason.
Just an idea.
I agree with your sentiment entirely. It would be possible to craft a logical, and satisfying storyline where a new enemy who was nowhere near as powerful as the Reapers, took advantage of the galaxy's weakened state.
But that's no good for marketing purposes. Marketing always seems to dictate that the new enemy be 10x more deadly than the old (and probably the 'creator' of the old, or something similar).
Another marketing strategy that I dislike, which is related, is to always be claiming the current player character is the most important person ever to exist in the whole wide universe. Kind of like how Bioware advertised Hawke. It's nearly always a letdown once they release the game...