If you allow Fist to live through Mass Effect 1 and then meet him in Mass Effect 2 on Omega, you will meet him a third time at the Talon Base during Omega DLC. He is completing a reluctant redemption arc by joining the Talons to avoid the Cerberus agents that are hunting for him.

If you allow the hanar prophet to keep preaching on the Citadel in Mass Effect 1, you'll see him again during the Citadel: Hanar Diplomat mission. He will be surrounded by a group of NPCs that grows larger the deeper into the game you play the mission.

The Consort will give you a trinket if you complete Citadel: Asari Consort, and if you later visit the prothean ruins on Eletania you'll activate "The Vision". If you managed to do this, then on Priority: Mars a datapad will change to reflect the knowledge gleaned from this vision.

A datapad on Priority: Mars will change depending upon the outcome of the Feros mission. If the colony survives, the martian base will be eager to see the artefacts they have dug up at the colony, but with less desirable outcomes, the tone will change severely.

Find the Odd Skull easter egg during Mass Effect 1, and in ME3 you'll find it again in Dr. Brysons laboratory. If you interact with it in there, you can then make use of it in your apartment as one of the new Statue decorations.

Should you choose to save the Council, the trees in the Human Embassy will change to red maples, or at least a tree that feels similar.

Additionally, before the Citadel Coup C-Sec will be a lot more relaxed and will not be armed. This is to make it a counterpoint to the ME2 dialogue that says a dead Council led to the humans militarising C-Sec.

If you chose to sacrifice the Council, then C-Sec in the Embassies will be armed across the entire length of the story (Phase 1 done so far). In future, this will be rolled out across the entire Citadel, as Mass Effect 2 dialogue makes it clear that the humans have been militarising C-Sec in a dead-Council galaxy.

If you have completed UNC: Asari Writings then you will be able to read an additional datapad on Kallini: Ardat-Yakshi Monastery which references the wisdom found within them.

If you did not attempt Grunts loyalty mission, then Gatatog Uvenk will be present among the krogan at the Hollows. Similarly, if you did not complete Mordin's loyalty mission, then Chief Weyrloc Guld will be visible.

If you rescue the scout Urdnot Darg from the Weyrloc lab during Mordin's loyalty mission, he'll appear as the krogan scout in Priority: Tuchanka for a brief cameo.

If you send Legion to the Illusive Man without speaking to him then the platform will appear as an enemy during Priority: Cerberus Headquarters. It will be named "Geth".

Garrus' fate changes the datapads on Priority: Palaven to reflect how his knowledge helped them to prepare for invasion. If he is alive it will paint them as being more ready for the battle. Additionally, the lunar defences will be more advanced, and will include anti-air turrets that are keeping the harvesters at bay.

Garrus' fate changes the datapads on Priority: Palaven to reflect how the turians were unprepared for the invasion. If he died, their organisation appears to be more chaotic, and without his defences, the harvesters are able to swarm the turians without challenge.

During Priority: Mars, the star that The Illusive Man is watching will be blue. Just as it was in the Mass Effect 2 ending.

Cerberus will be much further along with their experiments into indoctrination if you let them keep the base. For example, on Priority: Horizon you will see Abominations in the conversion tubes.

During Priority: Mars, the star that The Illusive Man is watching will be red. Just as it was in the Mass Effect 2 ending.

Completing these sidequests will change the composition of the forces that are trying to reclaim Omega from Cerberus. You do not need to complete all three to witness these changes, but you'll only see those factions you recruit.

If Kahje falls to the Reapers, then the refugee area will fill up with hanar before Priority: Citadel II. This is to help visualise that the hanar are struggling and fleeing to the Citadel for help.

If the hanar are saved, then you will see hanar in the hospital helping to tend to the wounded.

Before you start this quest, Purgatory is a shadow of its former vanilla self. Though still welcoming, many of its holograms are deactivated. Once you complete this sidequest, then not only will the vanilla hologram decorations will be fully restored, but additional holograms will be present.

If you complete Dekuuna: Elcor Evacuation, you will find a load of elcor in the refugee area.

After the invasion of Irune, the refugee area will fill up with volus that managed to escape the horrors of their homeworld.

In the immediate aftermath of the Citadel coup, you will find a Citadel struggling to recover from the attack. Areas will be blocked off for investigations, corpses and injured people are everywhere, and there is a sense of devastation in the air. On future visits, you will find that over time the Citadel begins to recover from the invasion.

Additionally, the pre-coup Docking Officer is replaced as she was killed in the opening cutscene of Priority: Citadel II.

Coming patch 1.4.0.

If you can solve the mystery of the solar panels and find the martian rover, you'll loot it ready for display in your apartment on the Citadel. This easter egg is hard to find.

If the geth survive Priority: Rannoch, or you manage to bring peace to the two races, then you will have a chance of seeing geth ships drifting past the window in the docking bay.

Additionally, you will see a small number of geth on Kithoi Wards 'Silversun Strip'. They will not be found in the casino.

If the quarians survive Priority: Rannoch, or you manage to bring peace to the two races, then you will have a chance of seeing quarian ships drifting past the window in the docking bay.

Additionally, you will see a small number of quarian on Kithoi Wards 'Silversun Strip'. They can also be found in Purgatory.

If you have played Mass Effect: Andromeda then you will see an advertisement for the Andromeda Initiative on the Citadel during Phase 1. It will not be there much longer after that first Citadel visit.

If you have read Mass Effect: Ascension there will be an additional datapad on Grissom Academy that discusses Cerberus attempts to capture Gillian Grayson. If you have not read the novel then you won't see it.

If you have played the mobile phone game Mass Effect: Infiltrator, then there will be a new terminal to read in the Citadel Holding Area. Reading this terminal will grant you the war asset that you once would've been given for completing that phone game. If you have not played the game then you won't see it.

NB: the value of the war asset has been severely reduced to prevent changing the balance of the EMS system.

If you have read Mass Effect: Retribution there will be an additional datapad on Priority: Horizon that discusses the torture of Paul Grayson in the novel. If you have not read the novel, you won't see it.

Finding the Governor Grothan Pazness war asset in EGM will mean that he'll show up in the Citadel Embassies after the Cerberus Coup. For now he cannot be interacted with, but in the future its a possibility.

If you have imported a save where you played LE1DP, then the c-sec executor will be Chellick from Mass Effect 1. If not, a nameless human executor as per vanilla ME3.

A very small number of Virtual Aliens will be visible on the Citadel after Priority: Thessia.

Buying the LOKI licence in Spectre Expansion Mod will allow you to see LOKI mechs bolstering C-Sec forces in the Citadel hub areas.

If you have won the support of the Lystheni then you will see a small number in Aria's bunker. They will not speak to you.