I'd like to piece together all the outcomes of player decisions in the ME series. I'd like to make this thread about players asking questions like, "what if I choose that instead?" Obviously everything in here is spoiler, so read at your own discretion.
Please do not come here and say, "Nothing matters, the endings are all the same!" While I, myself did not like the endings either, that's not what I'm here to talk about. I will also try to keep this thread semi-updated.
1) What happens if Garrus or Tali in the Omega 4 relay in ME 2? Who shows up on the moon of Palaven? Who shows up on the flotilla? Do you get a token turian to talk to/ join your party?
2) What happens if you killed the Rachni in ME 1? Do the Rachni mobs not show up in the game? Is there still the mission to investigate with the Grunt's Krogan group before curing the genophage?
3) What happens if your Fem Shepard romanced Jacob, does he still marry Brynn?
4) What happens if Miranda did not survive Omega 4, do you still try to save her sister on the Cerberus base?
5) What happens if your Fem Shepard romanced Thane? Does he still die? Is the dialogue really different during the death scene?
6) What happens if Mordin AND his student died in ME 2? Is there someone there to cure the genophage?
7) What happens if you gave Cerberus the reaper? How is the dialogue different?
8) What happens if you destroy all the Geth in ME2? How does the conflict with the Geth and Quarians play out?
Mass Effect 1
1) Council - For whatever reason, the council in ME 3 is the same regardless of whether or not you sacrificed them in the Sovereign attack. Udian is also the counselor even if Anderson was in the 2nd game. From what I've seen, there's only 1 line of dialogue difference, and they never explain why Anderson isn't on the council.
2) Rachni - I don't know what happens if you killed the Rachni (q2). Saving the Rachni, you will run into Rachni mobs (1 giant eye, 2 small blue lazers shooting a bazooka like shot with 3 sacks of spiderlings) who have been taken over by the reapers. You will also get a mission before curing the genophage to investigate the Rachni. On that mission you reunite with Grunt if he survived Omega 4. With him, you fight through the reaper/rachni cave and find that the queen was captured and enslaved by the reapers. You can free her, and she will join your fleet (worth about 100 -> 200 fleet str), or you can leave her to die. Grunt will survive regardless of your choice, but his company will take a few (10ish) more losses.
3) Wrex - If Wrex is alive, he becomes the leader of the Krogans and clan Urgnot. If Wrex died in ME1, you deal with his half brother Wreave. Both will demand that you cure the genophage before helping the Turians. Wrex will promise to try to make peace and not seek out revenge after the war. Wreave promises to wage war against the Turians and Salarians after the Reapers are gone.
4) Ashley vs Kaiden - Obviously, one of them survives and joins your team. Some of their dialogue is shared word for word, mainly on missions. All their dialogue when you talk to them between missions is different. Kaiden really seemed dull and underwritten in this game, but I didn't particularly like Ashley's conversations either.
5) Zhu's hope - Honestly, I don't even remember what happened there, but it is mentioned in 1 line when invading the Cerberus base.
Mass Effect 2
1) Miranda - If Miranda survives, you will get an email from her to meet at the citadel. You can choose to re-ignite the romance. This blocks you from starting other romances. She will then talk to you again later at the spectre terminal near the HU embassy. Finally, you'll get an email to meet her in the apartments for a quickie. If you meet her in the apartments, she will not die on to Kai Leng or her father at the Cerberus base, and you get +25 fleet str.
2) Jack - If Jack survives, you will meet her on the side mission at the school for gifted biotics. She teaches there now. After the mission you can meet her at the club on the Citadel. There, you can re-ignite your romance, which blocks others. Whether or not you romance her, she joins her students fighting for the fleet, +25 str. If she died on Omega 4, she simply doesn't show up at the school. The main male student dies as you're running to the shuttle trying to save Rodriguz.
3) Garrus - I don't know what would happen if Garrus died in ME2. If he survives, he shows up when you start working with the Turians, and joins your party. He has lots of great dialogue, and jokes when you find him between missions (trades jokes with Joker, Epeens with James Vega, makes out with Tali).
4) Tali - I don't know what would happen if Tali died in ME2. If she survives, she shows up when you start working with the Quarian, and joins your party. You can lose her if the Quarians are wiped out.
5) Grunt - If Grunt survived the Omega 4 relay, you meet him when investigating the Rachni. He's now in clan Urgnot, and leads his own commando unit. He will survive the mission regardless, and gives +25 fleet str.
6) Legion - If Legion died on Omega 4, you interact with another unit, simply named "Geth." They look exactly the same, and even have the same hole blown out of their chest piece. The dialogue is rather different however, and you miss him calling you "Shepard Commander." Both will try to upload the reaper tech to their fleet. Legion and Geth will die, regardless.
7) Thane - If Thane survives, he will show up in the Hospital promising to watch over Ashley or Kaiden. You get to have a nice chat with him. He also foils the assassination attempt during the Citadel, but is mortally wounded. He also provides a nice death scene in the hospital with his son later on. If Thane did not survive, he doesn't appear in the hospital. Kai Leng will kill the Salarian Counselor during the attack on the Citadel, and Udina will have a video recording to frame you for the murder. It puts some more doubt in Ashley/Kaiden, but you get to kill Udina regardless.
8) Mordin - I don't know what would happen if Mordin died. He shows up trying to cure the genophage. Wrex or Wreave, cure or sabotage the genophage, as far as I know, Mordin dies in ME3 regardless.
9) Samara - I don't know what would happen if Samara died in ME2. She shows up on a side quest when you go to check on the Asari school. Apparently, all 3 of her daughters have that disease that kills a person after Asari mind sex. You will save 1 of the daughters. Samara will then either commit suicide (rather than fulfill her duty as a justicar killing her last daughter), or you can convince her there's another way. She'll then join the front lines and +25 to your fleet.
10) Jacob - Jacob shows up in the N7 side mission Cerberus Scientists. I'd like to know what would happen if your fem shepard romanced him (q3). He's there protecting a group of scientist fleeing Cerberus. He falls in love with the main scientist Brynn. After the mission, you'll be able to talk to him at the hospital. There, you'll find out that they're having a baby, but they'll continue fighting till the reapers are gone. Jacob gives +25 fleet str.
11) Katsume - Katsume shows up on a Citadel side mission exposing a traitorous ambassador. After the mission, you can convince her to join the fleet. She will go around stealing rare materials for the construction of the Crucible, and give +25 to fleet str.
12) Zaeed - Zaeed shows up in a side Citadel mission where Cerberus tries to assassinate the Hannar Ambassador, he stops the assassination and joins the fleet for 25 str.
13) Maelon's data - From what I've seen, if you do not save Maelon's data from Mordin's loyalty mission, Eve will die on Tuchunga when Mordin extracts her DNA. After the cure, there will be a scene where you burn Eve's body that reminds me of the movie Troy. If you save Maelon's data, but sabotage the cure, the dialogue is slightly different, but Mordin still dies. Afterwards, the Krogan not knowing the difference, Eve and Wrex/Wreave will stand near a stone table and commend you.
14) Destroying/Saving the reaper - I only killed the reaper. Someone let me know what happens if you hand it over to Cerberus!
Mass Effect 3
1) The Genophage: Before embarking on the mission to cure the genophage, the Salarian Diatriss will ask you to sabotage the cure.
If you reveal the plans, Mordin will take the elevator up to the top, and fix the problem as the cure is released. The shroud then explodes, killing Mordin. The Salarians will never join your fleet.
If you don't reveal the plan, Mordin will detect something is wrong. If you try to stop him from going up, he'll figure it out. You'll then have a brief argument with him about it. To stop the cure, you have to shoot Mordin in the back. He still takes the elevator to the top, but dies before being able to fix the problem. The Salarian join your fleet, but you will lose the bulk of the Krogan later on. After the attack on the Citadel, Wrex/Wreave will figure it out, and confront you on the Citadel. You will either be forced to shoot Wrex/Wreave, or watch as Bailey and C-Sec kill them. Later on in a conversation with Garrus he will ask you why Wrex/Wreave would attack you, and I choose to tell him the truth, to which he responded something like, "Damn, a few years ago, I would've said the price is too high. I guess that's what this war has done to us."
2) The Quarians and Geth:
After freeing the Geth from the Reaper control, Admiral Gerrel will order the fleet to finish off all the Geth. Legion, or token Geth will then ask you for permission to upload Reaper tech to the Geth.
If you refuse to let him upload the tech twice, he will disobey and begin the upload anyways. Tali will then kill Legion/ token Geth. The Quarian fleet will then wipe out all the Geth. (It's a nice cutscene). The Quarians add something around 480 to your fleet.
If you allow him to upload the tech, Tali will plead with you not to. If you ignore her, Legion/Token Geth will upload the data. Legion/Geth will sacrifice themselves to do the upload. The Geth fleet will then wipe out all the Quarians (It's a different, and even nicer cutscene). Afterwards, Tali will commit suicide. The Geth will add something around 600 to your fleet.
If your reputation is above 2 bars, you can get a peace options. Allow Legion/Geth to upload the data. Then use your reputation check to call for peace/threaten to Quarians. The Quarians will break off their attack, and you will get both fleets to join you. Legion/Geth still die uploading the data.
I don't know if this makes a difference, but sometime while on your ship, you hear Admiral Gerrel and Raan having an argument (Gerrel is the one who will stop at nothing to kill the Geth). You can choose and support one of them in the argument. I definitely supported Raan on the playthrough where they made peace.
Feel free to ask any other "What If questions," and try to answer the ones posted if you can. Thanks!