Another interesting plot could be a pre - war story that shows the signs and warnings that were ignored before the Reaper War actually began. It could follow a group of scientists or military officers who were aware of the impending threat but were unable to convince the authorities to take action.
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.
A plot could revolve around a character who has the ability to communicate with the Reapers in a unique way. Maybe they try to negotiate a truce or find a weakness by getting inside the Reapers' minds. This would add a psychological aspect to the fanfiction.
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.
A great plot could be centered around a diplomatic mission during the war. Maybe a small group of humans and Turians who are trying to find a way to end the war before it gets out of control. They have to overcome their cultural differences and the distrust between their species. There could be a lot of internal conflicts within the group as well as external threats from those who don't want the war to end.
One idea could be to focus on a group of human soldiers who are stranded behind Batarian lines during the war. They have to rely on their wits and each other to survive and find a way back to their own forces. Maybe they form an unexpected alliance with a group of Batarians who also oppose the war for their own reasons.
Another plot could revolve around the diplomatic efforts during the war. Maybe there's a human diplomat who tries to establish communication with the Turians in the middle of the conflict. They face many challenges, like cultural misunderstandings and the resistance from both sides who are more interested in fighting. The diplomat could befriend a Turian counterpart who also sees the futility of the war and together they try to find a way to end it.
One idea could be that Naruto is somehow transported to the Mass Effect universe and has to face the Reapers. His ninja skills might give him an edge in combat against the Reapers' minions. He could use his shadow clones to confuse the enemies and his rasengan as a powerful attack. For example, he might be able to take out a group of husks with a well - timed rasengan.
A plot could revolve around a diplomatic mission during the First Contact War. Maybe a group tries to broker peace but faces many obstacles from both sides' extremists. This could explore the complex politics and cultural misunderstandings that fuel the war.
Liara T'Soni can also be a great character to feature. Her knowledge of the Protheans could be crucial in a fanfiction. Maybe she discovers a hidden Prothean weapon that could turn the tide of the war against the Reapers.
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 Shepard accidentally getting thrown into a dimension where the Reapers are already victorious. They have to find a way to team up with the remnants of different species to reverse the situation. Another could be a dimension where the Mass Effect fields work differently, leading to unique biotic and tech abilities that Shepard has to learn to use in order to get back home.
One idea could be that Naruto is somehow transported to the Mass Effect universe and his unique abilities draw the attention of various female characters. They see his determination and power as something new and exciting, leading to a harem situation. For example, characters like Liara might be intrigued by his chakra abilities which are completely different from biotics in their world.