"There's no way to tightly 'wrap up' something that has been accumulating and branching and growing for so long like that."
Well not with that attitude there isn't.
There's a difference between trying to wrap that stuff up in the final game, and just making up a magic super weapon in the final game to end things with.
ME1 already showed that major decisions don't always have to have major consequences. You let the council die and guess what? There's just a new council in ME2. And I forget this one, but I think I'd you kill the Rachni queen, a different Rachni queen is in ME3.
IMO the endings of ME3 should have just been (with slight variations to take account of your individual choices):
- You lose: Reapers win and the cycle begins again. They actually have that as an ending which I do appreciate even if its kinda hard to get without knowing how.
- You win, but just barely and the galaxy is mostly destroyed and the races aren't super friendly with each other afterward, and Shepard is probably dead.
- You win, you solved all the race's issues, and there's peace and prosperity for the the time being, Shepard maybe survives.
Sure it's a more straightforward ensing, but I kniw that would have made people happy (or at least not angry), it would have made the choices you made throughout the trilogy matter in the end, and it doesn't throw any weird shit at you at the last minute like controlling the Reapers or magically melding tech and organic life together.
You can look at Dragon Age Origins as an example of that working well. That game is kinda similar to ME3's story. Big evil monsters are coming, gather up allies by solving their issues, and then at the end you actually see them fighting alongside you and then the VERY end decision is still killing the big dragon, but you just choose how to do it: you kill it and die, Alister kills it and dies, loghain kills it and dies, you\Alister kill it but live by giving Morigan a god-baby.
Yes that last one is the equivalent of a magic space weapon, but at least in that game there's actual magic so it makes sense.