The way I see it, having the reapers around as guardians completely eliminates any chance of conflict because as we've seen, there's no force in the galaxy that can oppose them head on and live. That doesn't leave a lot of room for conflict on any scale conducive to drama. Not that I'd want the next story to be yet another massive epic war, but just having them there puts too many limits on what can be done.
I suppose they could set up that the reapers have mysteriously disappeared, but in that case it may as well be the destroy ending.
Hell, well that's the thing... IMO -- and this is coming from a huge fan of the series, mind you, it doesn't actually bother me too
much -- BioWare's developers are masters of folding seemingly big choices back into a main... plot threshold, if you will. So it wouldn't really surprise me. That's sorta what I was intending to imply, a little, is that it could be folded inward like that, whereas with Synthesis... I dunno, it'd still be possible, but they'd probably want to tone down a lot of the implications and explain that most races decided green wasn't their color.
It's not great storytelling practice, but it's not anything we haven't seen BioWare's writers do before, and they've rarely done so in completely idiotic ways.
A thought I had back before ME3 came out was that it'd be an interesting twist if when the reapers were finally defeated, it turns out their purpose was to preserve organic life in this galaxy from an external threat. Something even worse than they were. The thinking being that they weren't hibernating in dark space, but fighting a constant battle across the aeons and the harvesting cycles were the only way they could keep their numbers up against the sheer attrition of a billion year war. So by destroying the reapers, you open the milky-way up to be sterilized by whatever nightmarish force that had just barely been held in check by a hundred thousand reapers. Bit of a cheep "there's always a bigger fish" ending I know, but I liked the irony that the reapers really were actually acting in the interest of all life as they claimed.
That's cool, and kind if ties in with original series lead writer Drew Karpyshyn's idea, although I prefer yours to his, lol. Karpyshyn's initial thought -- and this is where that seemingly-abandoned (for now, at least) gig in ME2 with the quarians investigating dark energy came from -- was that the Reapers were preserving races and using their scientific diversity to spend the eons and eons required to figure out how to combat the build-up of dark energy in the galaxy that was threatening to destroy us all at a certain point.
It was a touch-and-go plotline, and the endgame choice really didn't interest me. It made the Catalyst gig seem positively jaw-droppingly good by comparison, IMO. (You had to choose whether to destroy the Reapers, or join them on their quest. I'd almost certainly choose destroy damn near every time in this instance, and it really makes the Reapers seem far softer than ME3's endings even did.)
I don't have a link offhand, but check out the Mass Effect Wiki for more information if you're interested. It's... I don't know, I really don't care for it very much and genuinely prefer what we were given, heh. But the way you tell it, I think it'd be potentially interesting.
As far as "there are no real threats after this" is concerned, well, I'm not a big fan of Star Wars literature as it seems to constantly try to raise the stakes to higher and higher, more and more absurd levels from what I've seen, but I wouldn't be completely opposed to Mass Effect's universe raising the stakes once.
Just once, lol. Some extragalactic threat, so long as it was handled far better than, "and just when the survivors thought they caught a break, this completely untimely attack from the Andromeda Galaxy killed Joker and destroyed the fledgling Galactic Alliance". Or something.
There's plenty of room for smaller-scale conflict in Mass Effect's future, too, and I'd gladly take that as well. Probably more so, yeah, but a 'bigger' threat could allow all three (out of four >_>) endings a shot in the dark of working.