Goku would be an obvious ally for Captain America. Goku is always eager to fight for justice and protect the innocent, which aligns well with Captain America's values.
Mercy from Overwatch and the Medic from TF2 would be likely allies. They both have a strong focus on healing. Mercy's ability to fly and revive teammates could be well - complemented by the Medic's Ubercharge ability. Another pair could be Tracer and Scout. They are both fast - moving characters. They could use their speed to scout out areas, steal intel or make quick strikes against the enemy.
I think Mercy from Overwatch and Medic from TF2 could be great allies. They both have a strong focus on healing and could share medical knowledge and techniques from their respective universes. Mercy's ability to fly and revive teammates could be combined with Medic's ÜberCharge in interesting ways during battles.
I think Klaus and Loki could be likely allies. They both have a sense of mischief and are often underestimated. They might team up to outwit their more 'righteous' counterparts. Klaus has his vampire powers and Loki has his Asgardian magic and trickery.
Inuyasha and Thor could potentially be allies. They both have a sense of honor and are very powerful warriors. Thor with his hammer and Inuyasha with his sword could fight side by side against powerful enemies. Also, Captain America and Inuyasha might get along. Captain America would respect Inuyasha's fighting skills and determination, and Inuyasha would see Captain America as a worthy leader.
Issei would be a likely ally. He's the main character in Highschool DxD and is always looking for ways to become stronger. Sasuke's combat skills and knowledge could be very appealing to Issei. Also, Rias Gremory could be an ally. As a powerful devil and the leader of her peerage, she would recognize Sasuke's potential and value his skills in battles against common enemies.
Jean Grey would also be a likely ally. Her telepathic and telekinetic powers are extremely powerful. She could communicate with Superman on a mental level and use her powers to support him in battle. Plus, she has experience dealing with powerful threats, much like Superman, so they could work well together.
I think Sans and Iron Man could be great allies. Sans is intelligent in his own way in Glitchtale, with his knowledge of timelines and shortcuts. Iron Man is a tech genius. They could combine their knowledge to create strategies against common enemies. Iron Man might be intrigued by Sans' magic - like abilities and try to find a scientific explanation for them. Sans, on the other hand, would be impressed by Iron Man's high - tech gadgets.
Well, in a 'Dbz Captain America crossover fanfiction', perhaps there's a threat that endangers both universes. Captain America's sense of justice and unwavering moral compass could inspire the Z - Fighters. They could learn from his strategic thinking. And Captain America could be amazed by the power of ki and the Saiyans' transformations. Maybe they start training together, with Captain America trying to keep up with the intense training of the Z - Fighters, and in the end, they save both universes.
Midoriya Izuku. He's the main character and has a strong sense of justice, just like Captain America. They could learn a lot from each other, with Captain America teaching Midoriya about his combat experience and Midoriya showing Captain America the unique Quirks of their world.
Hermione would be a very likely ally. She's smart and resourceful, and could help Harry figure out the best ways to use magic against the Chitauri. Ron might also be there, with his loyalty and fighting skills. Additionally, some of the more experienced wizards or witches from Hogwarts could potentially join in to help Harry.