Lara Croft and Commander Shepard would be an obvious and very interesting pairing. They are both strong leaders and adventurers. Lara's knowledge of ancient civilizations and her survival skills could complement Shepard's military leadership and combat experience in space battles. They could learn a great deal from each other.
Another interesting pairing could be Garrus and Lara Croft. Garrus is a skilled sniper in the Mass Effect universe, and Lara is also very proficient with ranged weapons. They could exchange tips on marksmanship and stealth tactics. Also, Garrus's loyalty to his cause and Lara's determination to uncover the truth about ancient mysteries could make for a great dynamic between them.
One possible plot could be Lara Croft discovers a Prothean artifact during one of her adventures. This artifact has a connection to the Mass Effect universe, perhaps it contains data about the Reapers. She teams up with Commander Shepard to figure out how to use this information to stop the Reapers or some other threat that is starting to cross over from the Mass Effect universe into her world.
Sakura and Liara would make an interesting pair. Sakura, with her medical ninjutsu skills, could collaborate with Liara, who is also very knowledgeable in the medical and scientific fields. They could exchange knowledge about different forms of healing and perhaps even come up with new techniques by combining their respective medical expertise.
Thane Krios and Darth Maul. Thane is a deadly assassin in the Mass Effect universe, and Darth Maul is a Sith assassin. It would be fascinating to see them in a battle of wits and skills. Thane's stealth and marksmanship against Darth Maul's lightsaber skills and Sith magic would make for an intense encounter.
For me, the pairing of Miranda and Emerald could be really exciting. Miranda's genetically - enhanced abilities and her strategic mind could be a great influence on Emerald. Meanwhile, Emerald could show Miranda some of the more underhanded tactics used in RWBY's world. Also, Kaidan and Ren would be an interesting combination. Kaidan's biotic powers and Ren's ability to control his Aura could lead to some interesting experiments in combining their powers. They could also bond over their more reserved personalities.
Shepard and Captain America would be a great pairing. Both are leaders and have a strong sense of justice. Shepard could learn about Captain America's more old - fashioned values and leadership style from a different time, while Captain America could be impressed by Shepard's ability to lead in a diverse and technologically advanced universe.
Deku and Shepard would be an interesting pairing. Deku is a young hero - in - training with a lot to learn, and Shepard is an experienced leader. Shepard could mentor Deku, and Deku could show Shepard the power of having a quirk. Their different backgrounds and skills would make for great character development.
A crossover with the Assassin's Creed series could also work well. Lara Croft might find herself in the middle of the conflict between the Assassins and the Templars. She could learn about their hidden history and use their ancient techniques while still relying on her own methods of survival and discovery.
A rather unexpected but potentially great pairing could be Ashley Williams and Vala Mal Doran. Ashley is a tough and no - nonsense soldier in Mass Effect, and Vala is a wily and resourceful adventurer in Stargate. They could get into all sorts of scrapes together and use their different skills to get out of them. On the other hand, Kaidan Alenko and Jonas Quinn could be paired. Kaidan has biotic powers in Mass Effect, and Jonas has his own unique abilities in Stargate. They could explore how their powers might interact or complement each other in different situations.
A great Mass Effect crossover fanfiction idea could be a crossover with the Stargate universe. The Mass Effect races could discover the Stargate network and start exploring other galaxies through it. There could be conflicts and alliances with the various races in the Stargate universe. Also, a crossover with the Dragon Age series might be interesting. The mages and warriors of Dragon Age could interact with the biotics and soldiers of Mass Effect. They could learn from each other's magic and technology, and perhaps even fight together against a dark force that threatens both universes.
Well, perhaps in the fanfiction, Lara Croft is exploring an uncharted planet when she stumbles upon a Halo ring. This attracts the attention of the Covenant, and Master Chief arrives to deal with the situation. At first, there's some tension between them as they come from different worlds and ways of doing things. But as they start to understand each other's motives and skills, they combine Lara's knowledge of ancient mysteries and Master Chief's combat expertise to fight the Covenant and figure out the secrets of the Halo ring.