A Turian general could be a main character. Turians are known for their military discipline. This general could be in charge of coordinating the Turian forces during the war. He might have to make difficult decisions, like whether to sacrifice a smaller outpost to save a larger military installation. His character arc could involve learning to work with other species more effectively as the war progresses.
An Asari commando could be a main character. With her biotic powers, she would be a formidable fighter. She could be part of a special forces unit that undertakes dangerous missions behind enemy lines. Her story could explore how she uses her biotics in creative ways during combat, and also how she deals with the moral dilemmas that come with war, such as when to use lethal force against an enemy.
Shepard could be a main character. Given their experience in uniting different species and facing great threats, they would be a natural fit in a world war scenario. They could lead a coalition against a new enemy.
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.
In a Mass Effect World War 1 fanfiction, the different alien races from the Mass Effect universe could be involved. For example, the Salarians, known for their intelligence, could be advisors to one of the warring sides, providing strategic insights. They might use their knowledge of technology and warfare to try and influence the outcome of the battles, much like how some countries had foreign military advisors during World War 1.
A human scientist could also be a main character. The scientist could be part of a team studying the Turians' technology during the war. They might accidentally discover a way to end the war but have to convince both sides to trust them. Their knowledge of science could be the key to peace, but they face a lot of opposition from military leaders on both sides.
A human soldier who discovers an ancient Mass Effect artifact during the war. This discovery gives him some kind of special power or knowledge that he uses to his advantage on the battlefield.
Shepard from Mass Effect could be a main character. They could lead the XCOM forces in a joint mission against a common enemy. Their leadership skills would be very useful in coordinating the two very different groups.
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.
I'm not entirely sure about all the main characters specifically, but it would likely include human military leaders and Turian commanders as they are the two sides at war.
Shepard is often a very popular character in 'the world game mass effect fanfiction'. Writers like to explore different versions of Shepard, such as a more morally ambiguous Shepard or one with different backstories. Liara is also popular as she has a rich backstory and interesting abilities, and fanfiction writers can expand on her relationship with Shepard or her own personal growth.
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.
The Asari Commanders are often quite memorable. Their long lifespans give them a unique perspective on the war, and their biotic abilities make them formidable on the battlefield.