Garrus Vakarian and a Titan Guardian. Garrus is a great sniper, and Titans are known for their strength and front - line combat. They could learn from each other's combat strategies and form a strong bond while fighting side by side.
Shepard and Cayde - 6 would be an interesting pair. Shepard is a skilled military leader, and Cayde - 6 is a witty and resourceful Guardian. Their different personalities and combat skills could lead to some great banter and teamwork.
Liara T'Soni and Ikora Rey. Liara is a brilliant Asari scientist, and Ikora is a powerful Warlock. They could exchange knowledge about their respective universes' technologies and magics, which would be really fascinating.
Pairing the Witch Queen from Destiny with Darth Vader would be an intense combination in a 'Destiny Star Wars crossover fanfiction'. The Witch Queen's powerful magic and her manipulative nature against Darth Vader's dark side powers and his loyalty to the Emperor. It would be a battle of wills and a struggle for power as they either try to outwit each other or find some common ground.
I think Cayde - 6 from Destiny and Han Solo from Star Wars would make a great pair. Cayde - 6's humor and rogue - like nature combined with Han Solo's smuggler attitude would create some really fun and interesting interactions. They could get into all sorts of adventures together, whether it's smuggling goods or fighting against a common enemy.
Shepard from Mass Effect would be really interesting in Dragon Age. His leadership skills and combat experience would be a huge asset or a big threat depending on how the story goes. Also, Morrigan from Dragon Age could be fascinating in the Mass Effect universe. Her magic and manipulative nature would be something new in that world.
One idea could be to have the Guardians from Destiny encounter the Normandy crew in Mass Effect. They could team up to fight a common enemy like a new, powerful alien race that has the technology to disrupt both the Traveler's Light in Destiny and the Mass Relay network in Mass Effect.
Alex Mercer would be the most interesting. His abilities are so unique. In the Mass Effect universe, he could completely disrupt the power balance. He can shapeshift into anyone, which means he could infiltrate the Citadel Council or any other important organization with ease. His superhuman strength and regenerative abilities would also make him a formidable force against the enemies in Mass Effect, like the Collectors or the Reapers.
I think a pairing between Ben and Tali would be really cool. Tali is a brilliant engineer, and Ben's Omnitrix is a high - tech device. She could help him understand and maybe even upgrade it, while he could protect her with his alien powers during their adventures.
Commander Shepard would be a great character to include. His/her leadership skills and combat experience could be put to the test in the brutal and unforgiving Warhammer 40k universe. Maybe facing off against a Chaos warband or trying to negotiate with an Inquisitor.
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.
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.
Another plot could involve a shared enemy that exists in both universes. The combined forces of Mass Effect races and Destiny Guardians have to figure out how to defeat this enemy. They would need to overcome their differences in culture and combat styles to be successful.