It has ended. The last chapters wrapped up the plot and brought the manhwa to a close. There might be spin-offs or related works in the future, but the main storyline is completed.
Issei would probably be the first to want to fight Thanos. He's always eager to prove himself and protect his friends. He might initially underestimate Thanos due to his own growing power and confidence, but soon realize how powerful Thanos really is. Rias would be more cautious. She would likely try to gather information about Thanos first and then formulate a plan to either fight or negotiate with him.
Issei, being the hot - blooded character he is, would probably be excited to meet someone as powerful as Naruto. He might challenge Naruto to a friendly spar at first, hoping to learn from him and test his own strength. And as he gets to know Naruto better, he could start to see him as a brother - in - arms, especially if they face common enemies together.
It's a story that involves intense battles among high school students. They compete in a martial arts tournament where they display their unique skills and powers.
Rias would be devastated. Issei was not only her servant but also someone she had deep feelings for. She would likely go into a rage at first, vowing to find and punish whoever was responsible. Akeno would be heartbroken as well. She and Issei had a special bond, and she would be in mourning for a long time. Kiba might feel a sense of guilt, wondering if he could have done more to protect Issei. Asia would be in shock and constantly pray for Issei's soul, hoping for some kind of miracle.
Issei would probably be both scared and excited. He's used to dealing with powerful beings, but the Ghost Rider is on a whole different level. He might initially try to fight him out of bravado, but soon realize he's outmatched. Rias would be more cautious. She would see the Ghost Rider as a potential threat to her peerage and would try to figure out his motives before taking any action.