Another plot idea could be a love story set within the context of the Mass Effect universe. Maybe a human and an alien species fall in love, but they face numerous challenges due to the differences in their cultures and the political tensions between their respective races. They have to fight against prejudice and political intrigue to be together.
Some fanfictions might explore what would happen if a character had a different alignment. For example, if a normally good character turned bad. Or they could focus on a relationship that was only hinted at in the game and expand on it in a really detailed way. There could also be a plot where the Reapers have a different origin or motive than what was presented in the game.
A possible plot could involve a world war that starts due to a false - flag operation. One faction in the Mass Effect universe frames another for an attack on a neutral planet. This leads to a chain reaction of alliances forming and battles erupting across the galaxy. The Krogan, known for their strength and warrior culture, could be caught in the middle, deciding which side to support based on their own interests.
One idea could be exploring the political intrigue within the Terran Empire. How different factions vie for power and influence. Another could be the discovery of a new alien species by the Empire and how they handle it, whether through diplomacy or force.
One idea could be having the XCOM soldiers use their combat skills in the Mass Effect universe. Maybe they are sent to help fight against the Reapers. They could introduce their unique tactics and weaponry to the Mass Effect races.
One idea could be the integration of the mobile suit technology from Gundam into the Mass Effect universe. For example, Shepard's team could discover a lost Gundam unit and try to figure out how to use it against the Reapers. Maybe the advanced beam weaponry of the Gundam could be a game - changer in the fight.
One idea could be a story focused on a small, rag - tag group of survivors who have to band together to take on a Reaper - controlled outpost. They might not be soldiers, but ordinary people forced into extraordinary circumstances.
One idea could be exploring how Ashley's pregnancy affects her role in the military within the Mass Effect universe. Maybe she has to take on different missions or deal with the reactions of her superiors. Another could be focusing on her relationship with the protagonist during this time. Does it become stronger or face new challenges?
One idea could be that Venom's symbiotic nature interacts with the biotics in the Mass Effect universe. Maybe the symbiote gives a character enhanced biotic abilities that are unique and unheard of. For example, instead of just pushing or pulling with biotics, they can create a sort of living shield made of the symbiote's substance.
One idea could be that Naruto, with his unique abilities like the Rasengan and shadow clones, gets transported to the Mass Effect Andromeda universe. He could use his ninja skills to help the Pathfinder team in combat situations against the Kett or other enemies.
A possible plot is that a Mass Effect technology is introduced to the Naruto universe. Maybe a Mass Relay is found in the Hidden Leaf Village. This could lead to a whole new set of adventures as the ninja try to figure out what this strange technology is and how it can be used. Or, characters from both universes could be combined. Like a Turian with Sharingan abilities.