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.
Another possible pairing could be the demons from Doom and the geth in Mass Effect. The geth are a synthetic race, and the demons are otherworldly and malevolent. A story could explore how the geth would react to the demons' unrelenting aggression and if they could find a way to combat them together or separately.
Liara T'Soni and the Doom Slayer could also be an interesting pair. Liara is a very intelligent and curious asari, and she would surely be intrigued by the Doom Slayer. Her knowledge of ancient artifacts and technology could potentially tie in with the Doom Slayer's origin and his connection to the hellish forces in Doom.
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.
Definitely Commander Shepard from Mass Effect. He/She is the central character in Mass Effect and would be a natural fit to interact with XCOM forces. Also, XCOM's Central Officer or top - ranking soldiers could be prominent. They could be the ones making decisions on how to deal with Mass Effect threats or form alliances.
Well, Clarisse might be paired with a more aggressive Doom character. She's a fierce warrior in the Percy Jackson series. They could form a powerful duo, fighting side by side against the common enemies. Also, Tyson could be paired with a strong, hulking character from Doom. Tyson is a Cyclops with great strength, and finding a counterpart in Doom that shares that physical power would create an interesting dynamic in the fanfiction.
Percy Jackson and Carter Kane would be a great pair. Percy is used to the Greek mythological world and its challenges, while Carter has a deep understanding of Egyptian magic and mythology. Their different perspectives could lead to some interesting teamwork, like when they face a monster that has aspects of both Greek and Egyptian mythology, such as a Sphinx with the ability to use water attacks like a Greek hydra.
Harry Potter and Nate Ford could be a likely pair. Harry's experience with facing great evils and his leadership skills combined with Nate's strategic mind in planning heists would make for an interesting dynamic. They could learn from each other's methods of problem - solving.
Master Chief from Halo and Apollo from Battlestar Galactica could be an interesting pair. Master Chief is a super - soldier with incredible combat skills, and Apollo is a skilled pilot and military leader. Their different skillsets could lead to some great teamwork in a crossover story. Another possible pair could be Cortana and Starbuck. Cortana is an AI with vast knowledge, and Starbuck is a brash but talented pilot. They could have some really interesting exchanges, with Cortana providing strategic advice to Starbuck.
Buffy and Luke Skywalker could be an interesting pair. Buffy is a strong fighter and leader in her world, and Luke is a key figure in the Star Wars universe. They could learn from each other's combat styles and leadership skills.
In Doom Mass Effect crossover fanfiction, the technology aspect is fascinating. The advanced tech in Mass Effect like biotics and omni - tools could be juxtaposed with Doom's hell - inspired weaponry. Maybe a story where Shepard gets access to some of Doom's weapons and has to adapt to their raw power.
Harry Potter and Ichigo Kurosaki are very likely to be paired. They are both the main protagonists in their respective series. Harry has his strong sense of justice and magical powers, and Ichigo has his determination and Soul Reaper abilities. They could form a great partnership to fight against common enemies.
The Space Marines from Warhammer 40k are often popular. Their super - human strength and combat skills would make for an interesting match against the various alien species in Mass Effect. Commander Shepard from Mass Effect is also a common choice. His/her leadership and combat abilities could be a great addition to the often dark and gritty Warhammer 40k universe.