Based on the description from the EA store it seems like Mass Effect 3 will be all about uniting the many galactic species against the Reapers. Who will be on your side when the Reapers come knocking on Earth's door?
This article includes plot spoilers.
Background: Back on Noveria, the station at Peak 15 was overrun with ugly insect-like creatures that look like they crawled out of a freshwater pond in Louisiana. Turns out these vicious beasts were Rachni, an ancient species that once threatened the safety of the entire galaxy until the Krogan stepped in and wiped out the population. Their extinction made their sudden presence on Noveria an unpleasant surprise, and when Shepard discovered a Rachni Queen in a holding cell there was an important decision to make: Let the Queen go, or kill her and once and for all destroy the Rachni species.
Why It Will Matter: For those who chose to save the Queen, an Asari diplomat awaited Shepard's arrival on Illium to relay a message of peace and alliance. So it appears that whenever the final Reaper confrontation goes down, the Rachni will be on Shepard's side. Not a bad ally to have.
So what if you killed the Queen? Well then, I guess you're S.O.L. Maybe you shouldn't be such a jerk next time.
Background: The Quarians were not always an enviro-suit-wearing race. Once upon a time, they lived peacefully on their homeworld of Rannoch on the edge of dark space. The world was strangely free of harmful viruses or bacteria, so when the Quarians' own invention, the Geth, turned against them they had to flee to dirty clutches of space. Wandering the galaxy in a caravan of ships called the Migrant Fleet, the Quarians' immune systems weakened over the years until enviro-suits became the norm.
In Mass Effect 2 Shepard visited the roaming Quarian headquarters and it was clear that the passengers were becoming restless with their gypsy lifestyle. Murmurings of war flooded the ship, and the Quarian leaders were divided among the decision to try and clear their former home of the Geth. Using Shepard's persuasiveness, you could either urge the Quarians to go to war, or tell them to hold off until the Reaper threat subsided.
Why It Will Matter: If the Quarians attempt to take back their home world, it could mean the extinction of the entire race as the geth are formidable opponents that don't sleep, drink, eat, or have sensitive immune systems. Even if the Quarians manage to prevail, it's doubtful that the remaining survivors will be able to assist in the effort against the Reapers.
If you quelled their thirst for war, on the other hand, you've likely gained the Migrant Fleet as an ally in the upcoming battle.
Background: Both Mass Effect titles offered a romance sub-plot, but the question is did you stay faithful to your original bed buddy or were you put off by their abrupt greeting in Mass Effect 2? (Dammit, Kaidan, why won't you let me love you?!) Deciding to stray was pretty easy, after all the temptations were plentiful: Miranda's bountiful assets, Tali's delicate admiration, Thane's quiet confidence and Jacob's hunky muscles…well, you get the point.
Why It Will Matter: The repercussions for cheating on your former love are unknown (though, is it technically cheating if you were on a break?), but I hope it turns into a full-on Jerry Springer episode. There needs to be hair-pulling, name-calling, or fist-throwing. Okay, maybe not. Still, there better be some reward for staying faithful, like perhaps a nice, simple wedding ceremony? Or they could just get it on again. Either way, there needs to be a fulfilling reunion in the final chapter.
Another thing to consider is that both Ashley and Kaidan represent the Alliance Navy. Since Shepard "went rogue" in Mass Effect 2 and joined forces with Cerberus, tensions have been high between the Commander and his (or her) former employer. Perhaps a good relationship with one of them will make it easier to ingratiate yourself back with that group. Or maybe it will just be about sex.
Background: The council is the governing authority over the happenings in Citadel space. Comprised of a Turian, an Asari, and a Salarian, the group was generally rude, borderline racist against humans, and a general pain in the ass to deal with in the first game. When Sovereign (the first Reaper ship discovered), launched an assault on the Citadel base the council fled to their escape ship, the Destiny Ascension. Under extreme fire from the supporting Geth troops, the council could either be left to their own devices of defense (i.e. they will die) or the Alliance Navy could swoop in and save the day, depending on your call.
Why It Will Matter: Let's be honest -- it was really tempting to just leave the council to die. They were total jerks. But without their political pull, the human race becomes more of a bully-dictatorship than a peaceful ally. This means it might be more difficult to get the Turians, Asari, or Salarians on your side.
If you elected to save their lives, the trio warms to you (they damn well better), and it will likely be easier to gain their respective species' aid against the Reapers.
Your decision of who should lead the galaxy (if the council died) or become the first human representative on the council, should also influence how easy it is to garner their goodwill. Captain Anderson, though more polite and without a doubt a better human being, lacked the political background of Ambassador Udina and therefore the skill to push human agendas.
Background: Urdnot Wrex is by far the most bad-ass character in the Mass Effect franchise. A true Krogan, his thirst for battle is evident, but he still has the common sense to know what's best for his currently withering species. After the Salarians inflicted a genophage to keep the overly-aggressive Krogan population down, the majority of Clan leaders still engaged in civil war, further ensuring their species' demise. Wrex, on the other hand, saw a better way: bunker down, band together, and focus on breeding until the population rises. Eventually he realized the majority of his brethren weren't ready for such a drastic change of mindset, so he returned to bounty hunting.
When Commander Shepard and crew landed on Virmire, they discovered a Krogan breeding ground designed to serve the Reapers. Wrex struggled with the idea of destroying the base, and therefore the cure to the genophage, but if you leveled up your persuasion skills you could talk the big guy down. Otherwise, you or Ashley put a bullet in his brain.
Why It Will Matter: If you saved Wrex from Ashley's trigger-happy finger, he shows up as the leader of the Krogan clans on Tuchanka. Grateful that you were able to talk him down, he assists you in your journey and even allows Grunt to become a member of Clan Urdnot.
If your speech level wasn't high enough to save him, or you just felt like killing something at that moment in time, you can expect that your mission to recruit the Krogan clans will be much more difficult. Also, what the hell were you thinking? Wrex is awesome.
Background: Everyone that's played Mass Effect knows that there are a butt-load of Reapers waiting out in dark space to come in and gobble everyone in the galaxy up. Or turn them into Collectors. They're difficult to kill and can Jedi-mind trick organic life. Basically, they're all-around bad news bears.
After defeating Sovereign (the one lone Reaper stationed outside of dark space to open up the relays and let the rest in for a big devouring festival), we ran into the Collectors in Mass Effect 2. Former Protheans that were altered beyond recognition, these bug-like aliens were slaves to the Reapers and building a new breed of Reaper modeled after humans. Creepy. Once you put down the baby Reaper, a drastic choice emerged: Save the base for "research" and hand it over to Cerberus or destroy it?
Why It Will Matter: Well, obviously the whole point of this game is kicking the Reapers' cold, metal asses. Either that or the entire universe gets destroyed. We better hope it's the Reapers.
The fate of the Collector base could be a huge factor in the outcome of the war. The technology could help to better arm troops against these sentient machines or it could completely backfire and turn more people into Reaper slaves.
Destroying the base means losing out on potential weapon upgrades, but it's also safer and ethically "the right thing to do."
Obviously this short list doesn't include everything that will be going on in Mass Effect 3. What plot lines do you think will appear in the third game?