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.
Another plot could involve Venom getting stranded in the Mass Effect galaxy. He has to team up with the Normandy crew. The crew is initially suspicious of Venom, but as they face various threats, they realize his power can be a great asset. For instance, Venom's ability to adapt and survive in different environments could help them during a mission on a hostile planet with extreme conditions.
A third plot idea is that the Reapers become interested in Venom. They see the symbiote as a new form of biological weapon or technology. Venom, not one to be controlled, battles against the Reapers while also trying to find a way back to his own universe. This could lead to some epic battles and interesting alliances within the Mass Effect universe.
One idea could be that Naruto is somehow transported to the Mass Effect universe. He could use his ninja skills, like his shadow clones, in combat against the various alien species in Mass Effect. Maybe he teams up with Shepard and they learn from each other's combat styles.
One idea could be that Naruto somehow gets transported to the Mass Effect Andromeda universe. He might use his chakra powers in unique ways, like using Shadow Clones to outmaneuver the Kett. Another plot could involve Naruto befriending Ryder and teaching them about chakra, which then gives Ryder an edge in combat and exploration.
One idea could be that Revan, with his unique combat skills and Force abilities, gets involved in a mission in the Mass Effect universe. Maybe he teams up with Shepard to fight a common enemy that has some sort of connection to the dark side of the Force. Another could be that Revan's knowledge of ancient technologies in the Star Wars universe helps the Mass Effect races to upgrade their own tech, creating new dynamics in their galactic politics.
One plot idea could be that Rex's nanites start to interact strangely with the eezo in the Mass Effect universe. This causes unforeseen effects like creating new forms of biotics or causing glitches in mass relays. Another idea is that Rex is mistaken for a new species and is studied by Cerberus, who want to use his nanites for their own evil purposes. He has to escape and team up with Shepard to take down Cerberus once and for all.
Some fanfictions focus on the idea of the symbiote being a sort of 'alien within an alien' in the context of Mass Effect. It could be hunted by Cerberus for study, thinking it's a new type of alien life form. The Venom symbiote's need for a host and the way it changes the host's personality could lead to some really complex and engaging storylines. For example, if a Krogan became the host, how would the combination of their natural aggression and Venom's influence play out? It could be a really wild and exciting read.
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.