One possibility is that Noble Six's Spartan abilities and training get integrated into the Mass Effect world. They could start training the Alliance soldiers in new combat techniques. And perhaps they could help in developing new technologies by combining the knowledge from their two worlds.
Well, the plot could involve Noble Six being somehow transported into the Mass Effect universe. Maybe they could team up with Shepard. Noble Six's combat skills would be a great asset in the fight against the Reapers.
Noble Six would probably be a bit of an enigma to the Mass Effect characters at first. They might be cautious around Six because of their different appearance and unknown abilities. But over time, they could learn to trust each other.
Well, it could be about Ben using his alien forms to fight alongside the Mass Effect characters against a common enemy like the Reapers. Maybe his Four Arms form could be used for brute strength in hand - to - hand combat with the husks.
One possibility could be that the races from Mass Effect come to the now - abandoned Earth and start exploring the ruins. They might find remnants of human technology and culture that could change their understanding of the humans. For example, they could discover old military bases with advanced weaponry blueprints that could be used in their own conflicts.
One possible plot could be Lara Croft discovers a Prothean artifact during one of her adventures. This artifact has a connection to the Mass Effect universe, perhaps it contains data about the Reapers. She teams up with Commander Shepard to figure out how to use this information to stop the Reapers or some other threat that is starting to cross over from the Mass Effect universe into her world.
Perhaps in the crossover fanfiction, the dark energy in the Mass Effect universe has some strange effect on Asura. It either amplifies his wrath to uncontrollable levels or gives him new powers. Meanwhile, the Asari, with their biotic abilities, try to form a connection with Asura to understand his nature. This connection could lead to a mutual understanding between Asura and the Mass Effect races, and together they work towards a common goal, like saving a planet that is on the verge of destruction due to a combination of Mass Effect threats and something related to Asura's past.
A possible plot is that the Reapers from Mass Effect start invading the world of My Hero Academia. The heroes would have to use their quirks in new and creative ways to fight off these giant mechanical threats. Maybe All Might could inspire the Mass Effect heroes with his unwavering spirit, while Shepard could teach the My Hero Academia characters about advanced military tactics.
One possible plot could be a crossover of the technologies. For example, the advanced biotics from Mass Effect could interact with the UNSC's military hardware. Maybe the Mass Effect races would be intrigued by the UNSC's more straightforward combat tactics compared to their own complex political and military strategies.
Well, in a Naruto banished Mass Effect fanfiction, the story could start with Naruto being wrongly accused in his world and sent away. Once in the Mass Effect setting, he could form alliances with different species. For example, he could team up with the Turians due to their military discipline which might remind him of the shinobi way. His jutsu could also be a source of great mystery and power in that universe, leading to various adventures as different factions either try to recruit him or eliminate him.
One interesting plot combination could be the use of Crysis' nanosuit technology in the Mass Effect universe. For example, Shepard could get ahold of a prototype nanosuit that gives them enhanced combat abilities, like increased speed and strength, which would be a huge advantage against the Reapers. Another could be the merging of the alien races from both universes. Maybe the Turians and the Ceph could form an uneasy alliance due to a common threat.
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.