It can be good as a shooter, but it will suck at story.
That entire thing of Reapers invading Earth screams of a plot deprived game. If they went more sci-fi, like the game was claimed to be, there would be some sort of ancient civilization that probably built the Reapers and in ME3 you would have to uncover their secrets and shit and the game would revolve solely on that.
The Reapers are the threat you must eliminate and for that you must unite the galaxy into doing so. Failure to gain certain allies would result in consequences as to how you stop the Reapers and what collateral damage would result from your actions...
Scratch what I just said. Let me lay out my view of what the ME series should have been all about.
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For a good series finale, ME2 couldn't have happened the way it did. ME2 should have been solely about finding out the means to stop the Reaper threat as everything pointed out by the end of ME1. Shepard said there was still much to be done regarding the coming of the Reapers and in ME2 he should have continued on that path, regardless of working for Cerburus or not. Sure, stop the colonies abductions if you will but instead of spending 80% of the time "building up the team" and doing unimportant sidequests for them, have your spare time dedicated into uncovering more about the Protheans or even older races for clues. The VI on Ilos was inoperative? SO WHAT!? There is an entire fortress down there with lots of technology ready to be salvaged and studied. ME2 should have been about doing that, finding out clues about the Reapers. Have that as the main quest instead of stopping the Collectors. Collectors should have been a secondary quest that still carries consequences into the future (ME3), but not as important nor vital as preparing yourself for the greater threat.
Then in ME3 you unite the galaxy with what you have accomplished in ME3. Your previous actions now reflect on who you can get at your side and at what costs. The game should be about that - doesn't matter if your discoveries led to a big fucking cannon of doom that tears a now hole into Reaper ships in one shot. At least you would have something plausible to work with. The meat of ME3 then is the aftermath. You get varied endings based on your actions from ME1 and 2 and who you managed to save in ME3. Have you, the player chose what planets/species to save in ME3. Also your odds of success are measured on what you accomplish in ME2 and your actions in ME3. Have the entire game be a full out galaxy wide war and all you do is play the galactic general, positioning battle fleets and picking where you will personally assault. You can't be everywhere so this leads to many different outcomes. Include failure as a possible result of your missions and then the Mass Effect franchise would be spoken highly by everyone that actually likes sci-fi.
That would be a mature, dark game in a good way. You are solely responsible for the fate of the galaxy and there are no easy choices to be made. You can also be selfish enough as to save Earth and fuck the rest (Pure Renegade), it's a valid approach.
But we all know that's not what gonna happen. ME3 will have something bizarre and incredibly retarded as a conclusion. You probably will find out the Reapers die to microwave ovens or wifi and that's it. Plot wise it will suck, I have no doubts about that. Gameplay can be interesting if they keep most of what they did in ME2 though and enhance it a little bit, not start from scratch.