John - 117 (Master Chief) is a must - have character. His iconic status in Halo would bring a lot of attention to the crossover. Garrus Vakarian from Mass Effect is also a fan - favorite. His sniper skills and loyalty could be put to the test in a Halo - influenced setting. Additionally, the Flood forms from Halo could be considered 'characters' in a sense. Their presence would completely change the dynamic of the Mass Effect universe, and seeing how the different characters from Mass Effect would respond to such a threat would be very engaging.
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.
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.
Popular characters in Halo Mass Effect Fanfiction include some of the elites from Halo. Their honor - based culture can create interesting interactions with Mass Effect characters. Thane Krios from Mass Effect is also a character that often shows up. His assassin skills and complex backstory can add depth to fanfiction stories. And then there's the Flood - infected characters which, while not traditional 'popular' in the sense of heroes, can create really dark and interesting plotlines in fanfiction.
Master Chief from Halo is also a common character in these crossovers. His super - soldier abilities and stoic personality could make for some interesting interactions with the more diverse cast of Mass Effect. He might be intrigued by the biotics or the different political systems in the Mass Effect galaxy.
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.
The 'Team - up Against a Greater Threat' trope is quite prevalent. For example, in Halo, different UNSC factions and even some Covenant remnants might join forces to face a new, more powerful alien threat. In Mass Effect, various species from different planets could unite against a new Reaper - like enemy. The 'Redemption Arc' is also seen often. A character who has made bad choices in the past, like a mercenary in Mass Effect, might try to make amends. In Halo, a disillusioned Covenant soldier could turn against his own kind and seek redemption.
One interesting concept could be the combination of the different alien species from both universes. For example, seeing how the Covenant from Halo would interact with the various alien races in Mass Effect like the Turians or the Asari. Another concept might be the technological mash - up, such as how the Spartan armor could be enhanced with Mass Effect tech or vice versa. Also, the idea of the Flood from Halo being a threat in the Mass Effect galaxy and how the different factions would deal with it is quite fascinating.
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.
Master Chief's super - soldier abilities combined with Ryder's biotic powers would be extremely powerful. Chief's physical strength, combat training, and strategic thinking, along with Ryder's ability to manipulate mass effect fields, could create a character capable of taking on the toughest enemies. For example, Chief could provide cover fire while Ryder uses biotics to lift and throw enemies.
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.