A plot could revolve around a character from Mass Effect finding a way to use biotics to control or neutralize Tiberium. Maybe a powerful Asari biotic discovers that her powers can manipulate the growth of Tiberium in a way that could save a planet from being overrun. This would bring in the unique elements of Mass Effect's biotics into the Tiberium Wars setting.
Another plot idea is a conspiracy within the Tiberium Wars universe that is being investigated by a Spectre - like agent from Mass Effect. This agent uncovers that a powerful corporation is using Tiberium research to create a super - weapon, similar to the Reapers in Mass Effect, but with the power of Tiberium. The agent then has to race against time to stop this weapon from being deployed.
Create a character who is an outcast from both universes. Maybe a Salarian who was exiled from his own people for his unorthodox views on Tiberium research, and then finds himself in the middle of a conflict in the Tiberium Wars. His knowledge of both Mass Effect technology and Tiberium could make him a key figure in the story, while his outcast status gives him a complex and interesting backstory.
Another plot idea is that the Mass Relay network somehow connects to hyperspace lanes in the Star Wars universe. This causes chaos as ships from both universes start popping up in each other's space. A group of Mass Effect characters and a group of Star Wars characters could come together to try and restore order and figure out how to separate the two networks again.
Well, you could write a fanfiction about a group of civilians caught in the middle of the Tiberium Wars. Show their struggle to survive in a world dominated by Tiberium and warring factions. Or perhaps a story about a scientist who discovers a new form of Tiberium with unique properties that could either save the world or destroy it. Another interesting angle would be to write from the perspective of a Tiberium - mutated creature and how it views the warring humans.
Another plot could involve the Mass Effect races coming to Earth. The Avengers initially see them as a threat but then realize they are also facing a common enemy, like a powerful alien force that is both a danger to Earth and the Mass Effect galaxy. So they join forces. For example, Garrus could team up with Hawkeye for long - range attacks.
One idea could be the Imperium of Man encountering the Citadel Council. The Imperium's strict dogma and military might would clash with the Council's more diplomatic approach. For example, the Space Marines might be seen as terrifying super - soldiers by the Mass Effect races.
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.
Well, in 'xcom mass effect crossover fanfic', perhaps a cool concept is the interaction of the characters. Imagine Commander Shepard teaming up with XCOM operatives. They could share their combat strategies and knowledge. Also, the idea of using Mass Effect's biotics in XCOM missions or having XCOM's psi - ops in the Mass Effect universe could add a whole new layer of excitement to the story.
One idea could be that a character from Mass Effect somehow gets transported to the Naruto universe and has the ability to mind - control certain characters, leading to the formation of an unexpected harem. Maybe they use this power to bring together ninjas with different skills to form a unique team.
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.