I've been thinking about this since finishing the second one and have a few ideas on how I think they will do it. This is all speculation, I could be way off on this stuff:
 
*The second game on a larger scale. While you will recruit party members, the main drive will be recruiting entire races to take on the Reapers.
*Decisions from the first and second games will have a great effect on your chances of recruiting certain races. Legion's loyalty mission for example will or wont allow you to recruit the geth.
*Living party members are also important, if Tali died for example, you will probably have a little difficulty with the Quarians.
*This structure will allow you to visit a number of the home planets of the various races and do missions for them. There will also be main plot missions like the second to take on the Reapers.
 *Final guess (possible spoiler): 
 **************************************
 The final mission will take place on Earth.

The final mission will take place on Earth. "
 

Difinitly hope it does NOT take place on Earth, that would just feel too generic.
 
I like the idea of recruiting races as well. Maybe there is a paragon/paramore type decision that involves recruiting races that are on the council or those that are not.

I can definitely see recruiting races and organizations.  Like if you did Aria's Patriarch Quest she would be easier to recruit, and if you kept the collector base intact then Cerberus will work cooperatively.  If you saved Tali from exile then the Flotilla will help, and if you saved the Rachni, they will help.  Stuff like that.

Dragon Age: Orgins but in space, that's the only possible way the story can go.

" @Superharman said:

The final mission will take place on Earth. "
 

I agree with some of that. It makes since from the setup of the second game. The last part I don't agree with. I would lean more towards it taking place in Dark Space, where the Reapers escaped to over and over again. I know it is a stretch but in the second game they destroy the Human-Reaper. What if there is already a Human-Reaper that has been around since before the first game. "

" @InKtOiD said:

" @Superharman said:

The final mission will take place on Earth. "
 

I agree with some of that. It makes since from the setup of the second game. The last part I don't agree with. I would lean more towards it taking place in Dark Space, where the Reapers escaped to over and over again. I know it is a stretch but in the second game they destroy the Human-Reaper. What if there is already a Human-Reaper that has been around since before the first game. "

The last game was all about preparing to go "where no one had gone before" (see what I did there? ugh) by chasing the collectors into the center of the galaxy. It was a great climax, but ending ME3 with a voyage into dark space to take on the reapers where they live is pretty similar. I agree that at some point we'll probably head into dark space, but I feel like it would be a much more intense experience to fight on Earth, the same way fighting in the citadel was so impactful in ME1. "

I agree with you on recruiting races.  I think getting the rachni, geth and curing the genophage for the krogans would be a really awesome way to structure the storyline

" Dragon Age: Orgins but in space, that's the only possible way the story can go. "

" @Simmse said:

" Dragon Age: Orgins but in space, that's the only possible way the story can go. "

This. "

You know, with all of the amazing places you go to in the Mass Effect games, I cannot help but feel that a simple "save the Earth" plot will do it for me.  The game already established enough for me to care about that galaxy as a whole, instead of Earth.  Nothing in the story made Earth so important in the first place.  If Bioware decides to just make Earth important with some cliche plot devices or reasoning, then it will be a huge disappointment.
 
That is not to say that the game won't be fun, but the story will most definitely be tainted with a lackluster set of conclusions if it does go this route I personally think.

Oh wow, this topic was from 5 months ago.
 
Also, I hope this game will be structured more like an RPG :o

Um, I think despite the fact that they have the ability to really branch out the ending, for the first time, they still won't. And just like the first 2 games, everyone's game will begin and end prettymuch identically. I think this because they probably have further plans for the "canon" and yet they still want to pretend your choices mean shit. It would be the bggest surprise in the world if they actually had divergent endings, Bioware aren't the type to take that kind of interesting step though.

I would say that rather to recruit races , it will be more about finding real evidence for the council about Reapers existence , because that would be easier (remember most races are in the Citadel) than redoing the second game again , not that I would mind ( I love ME2 ) , also I think that the game will be divided in acts  as in the final 1/3 of the game  will be the fight/resistance of the reapers' onslaught . Maybe the reapers will try to recruit other space faring races (  creatures like in the Lair of the Shadow Broker ) so we may have to fight them. Also we may need to find a way to effectively fight the reapers which = to finding more about them and their origin.

I'm guessing there will be a Medical/Healer class as Dr Chakwas survived no matter what.
You punch The Illusive Man in the face/ give him a hi-five.
A re-vamp of the Engineer class to make it more popular.
Soilder is not the default class.
Earth will be completely trashed up and overpopulated.
The shopkeepers on the citidel have an argument over which store Shepard is his/her favourite on the Citidel.
 
I guess this post is more about the class system and side storylines, but whatever

Fuuuuuck.... I hope it's not like ME2...

My guess is that they'll take ME3 as an opportunity to explore each race's homeworld, with an increased focus on the Asari and Turians in particular: that means trips to Palaven and Thessia. There'll probably also be something in the Perseus Veil to do with recruiting the Geth, perhaps based around the half-constructed Geth Dyson Sphere that Legion mentions. 
 
No real evidence for this other than it fits with the "Reapers going after homeworlds, starting with Earth" thing, and the fact that the latest ME book was heavily Turian-centric - even featuring female Turian characters. The second book (pre-ME2) set up Omega and the Flotilla, presumably because Drew Karpyshyn knew they were going to play a role in the second game. Wouldn't be surprised if that was the case now. 

I'd say it's going to be a matter of choosing sides in at least two major conflicts (Krogan VS Rachni, Geth VS Quarian) and bringing whatever's left to bear to fight the Reavers on Earth. I don't quite see this as "Dragon Age in space," but that seems to be the meme that's circulating. I think it's different enough since the previous two games have been steering in this direction, particularly ME2. 
I also hope they keep most of the characters from ME2, maybe let the ones from 1 rejoin, and hell, why not throw some new characters in while we're at it.

I agree there'll be a lot about the inter-species conflict, and it'll come down to easy route (pick a side, and lose a race) or hard route (diplomatic solution to recruit both). I'm hoping the decisions carry through the whole series, meaning having saved Wrex in ME makes getting Krogan on side is easier. 
 
But yeah, bet we don't see Earth until the end, gameplay-wise, but in tons of cutscenes. That VGA tease now looks like it'll cover the opening CG.

Earth gets attacked. You save Earth.

"Dragon Age: Orgins but in space, that's the only possible way the story can go. "

Just re-read my initial post after viewing the trailer and given how heavily they are pushing the attack on Earth, it should be interesting to see if this is the focus of the start of the game or the final conflict.  Perhaps there is an initial defence of Earth to begin the game which results in a signal being sent to the entire fleet and the bulk of the game takes place between these attacks.
 
My guess remains that you will be recruiting races to defend Earth at the games end. The structure of the story has meant that it is the Humans the Reapers are most scared off so their desire is to destroy humanity by taking Earth.