An Asari scientist could be a main character. She might be intrigued by the mystery of the abandoned Earth. Her knowledge of biotics and different cultures in the Mass Effect universe could help her in deciphering the secrets of Earth. She could be the one to make the first contact with any remaining humans or discover important artifacts. Also, a Krogan mercenary could be a main character. His brute strength and combat skills would be useful in exploring the dangerous and desolate Earth. He might be in it for the loot or just the thrill of adventure.
A Turian diplomat could play a major role. Since Turians are known for their discipline and order, he could be sent to investigate the situation on Earth. He might be trying to find out if the abandonment of Earth has any implications for the political and military balance in the Mass Effect galaxy. Additionally, a human survivor from the underground bunkers could be a main character. This person would have a unique perspective, having lived through the events that led to Earth's abandonment and now having to deal with the arrival of these alien species.
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.
Shepard could be one of the main characters. Imagine Shepard having to deal with Venom's presence. Shepard's leadership skills would be put to the test as they try to figure out if Venom is a threat or an ally. Also, Garrus could be involved. His combat skills and loyalty to Shepard would make for some interesting interactions with Venom.
Master Chief from Halo is an obvious choice. His super - soldier skills and stoic nature would be really interesting to see in the Mass Effect universe. Another one could be Commander Shepard from Mass Effect. Putting Shepard in the middle of a Halo - style conflict would be exciting. And Cortana, the AI from Halo, could also play a major role. Her intelligence and relationship with Master Chief could be explored in new ways when combined with the Mass Effect elements.
It could be a mix of characters from both universes. There might be classic WW2 soldiers like Patton or Eisenhower, but with a Mass Effect twist. Maybe they have some kind of exposure to Mass Effect technology that gives them special abilities. Also, there could be Mass Effect characters like Shepard who are somehow transported to the WW2 era.
Obviously, Tevos is a main character. She's a significant figure in the Asari government. And then there are the OCs (original characters). These can be a wide variety of beings. It could be a human soldier who gets involved in Asari politics because of some incident related to Tevos. Or an alien scientist who teams up with Tevos to study an ancient Prothean relic. Each OC brings their own backstory, skills, and motives to the story.
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.
The Doom Slayer and Commander Shepard are likely to be paired. They are both iconic heroes in their respective universes. Shepard is a strategic leader, while the Doom Slayer is a force of pure destruction. Their interactions could lead to some really interesting storylines.
Shepard from Mass Effect is a very likely character. Given their leadership skills and experience in dealing with various alien threats, they would be a natural fit in a crossover. They could potentially lead XCOM forces or at least be a key figure in coordinating between the two universes.
Garrus Vakarian might also be a common character. His relationship with Tali in the original games means that in a crossover, their dynamic can be explored further. He could be involved in protecting Tali or working with her in a new situation that the crossover creates.
Some of the main characters might be Shepard, as he is a central figure in the Mass Effect series. There could also be other important characters like Liara T'Soni or Garrus Vakarian, who are well - known and beloved in the Mass Effect lore and are likely to be featured in the graphic novel to some extent.