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.
In a 'Sasuke crossover Highschool DxD fanfiction', the most likely allies for Sasuke would include Gasper Vladi. Gasper has his own unique powers and is often in need of protection and guidance. Sasuke could provide that while also using Gasper's powers to their advantage in certain situations. Additionally, Sona Sitri might be an ally. She is a strategic thinker and would appreciate Sasuke's skills in combat and his ability to analyze situations quickly. Her calm and collected nature could also complement Sasuke's more intense personality.
One possible interesting plot could be Sasuke arriving in the Highschool DxD world during a time of chaos. Maybe there's a new threat that neither the devils nor the other factions can handle easily. Sasuke's ninja skills and Sharingan would give him an edge in combat. He could form an alliance with Issei and his group, teaching them some ninja techniques in return for learning about the supernatural powers in their world.
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.
Naruto and Issei are very likely crossover characters. Naruto, with his strong will and powerful ninja skills, would be an interesting contrast to Issei, the perverted but also determined protagonist of Highschool DxD. Another likely pair could be Sasuke and Vali. Both are brooding, powerful characters with their own agendas. Sasuke's Sharingan abilities could have some really cool interactions with Vali's powers in the context of this fanfiction. And of course, Kakashi could be a great crossover with some of the more strategic characters in Highschool DxD, like Sona, as he is known for his intelligence and combat skills.
Sasuke's character might become more open - minded. In the Highschool DxD world, he would be exposed to different cultures and ways of thinking. For instance, the concept of Devils and Angels having their own social structures would be new to him.
Iruka-sensei could be really interesting. He could be the kind teacher who always encourages Naruto and tries to get Sasuke to open up more. He might have some really cool lessons about the history of their ninja - like world in a modern high school setting, like teaching about the ancient ninja clans as part of a history class.
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.
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.
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.
Sasuke and Erza could be a likely pairing. Erza is a strong and disciplined character in Fairy Tail, much like Sasuke in his own way. They could bond over their combat skills and their determination to protect what they hold dear. Another possible pairing is Sasuke and Lucy. Lucy's kind - hearted nature might contrast with Sasuke's brooding personality, and their relationship could be one of growth and understanding for both characters.