A possible plot is that the OC's amnesia is caused by a powerful biotic attack. As they try to regain their memory, they discover that they have unique biotic abilities that they didn't know about before. They could then be recruited by Cerberus, thinking they are on the side of good, but as they start to remember more, they realize Cerberus's true motives. They might also have a connection to an ancient asari artifact that holds the key to both their past and the future of the galaxy, and different factions are vying for control of it.
You could create an OC species that is made up of energy beings. They don't have a physical form in the traditional sense but can interact with the physical world. Their society could be based on the transfer and sharing of energy knowledge. In the fanfiction, they might be hunted by other species who want to harness their energy for their own purposes, creating a conflict - ridden story.
One idea could be an OC who is a skilled biotic but has a unique way of using their powers, like being able to combine biotics with technology in unexpected ways. For example, they could use biotics to enhance the functionality of a broken piece of alien tech they find in Andromeda.
One plot idea could be Kasumi teaming up with an OC to uncover a long - lost Prothean artifact. The OC might have some unique skills or knowledge that complements Kasumi's stealth abilities.
One common theme is self - discovery. Since the OC has amnesia, they are on a journey to figure out who they are. Another theme could be relationships. They might have to rebuild relationships with characters they knew before losing their memory. Also, mystery often plays a big role. There's the mystery of what caused the amnesia in the first place, and what secrets their past holds that they need to uncover.
She could be on a quest for revenge against a powerful mercenary leader who wronged her in the past. This could lead her across different planets in the Mass Effect universe, facing various challenges and enemies along the way. During her journey, she might also uncover a larger conspiracy that the mercenary leader is involved in.
One popular plot idea is the journey of an OC soldier rising through the ranks in the Alliance military. They might face challenges like discrimination due to being an OC in a world dominated by established characters. Another idea could be an OC soldier on a secret mission to uncover a conspiracy within the Citadel Council. They could encounter various alien species and have to use their combat skills and wits to survive.
One popular plot idea is an OC joining a team on a dangerous mission to explore a newly discovered planet. They face all kinds of challenges like hostile alien life forms and environmental hazards. Another is an OC getting involved in the political intrigue between the different species in the Citadel. They might have to navigate through lies and power struggles. And a third idea could be an OC trying to solve a mystery related to an ancient Prothean artifact that has suddenly emerged.
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.