Tony Stark could also be a key figure in some fanfics, even though he's no longer alive in the main MCU timeline. His intellect and previous experiences with multiversal - related events (like in Endgame) could be explored in the fanfic. Maybe the Avengers find a way to access his knowledge or even a version of him from another universe to help them out.
For Naruto side, Naruto himself is very likely. His determination and growth arc can be interestingly combined with DC elements. Also, Sasuke with his powerful Sharingan could have some cool interactions in the DC universe.
Iruka Sensei could also be a prominent character. He was an important figure in Naruto's early life. In a second - chance story, he might play a role in guiding Naruto in new ways, or perhaps Naruto could use his second chance to repay Iruka's kindness in a more significant manner.
Sasuke would be a likely character. He already has a dark past and a tendency towards brooding and seeking power, which would fit well in a 'world of darkness' setting. Orochimaru is another one. His experiments and his general amorality make him a natural for a darker Naruto story. And of course, Madara Uchiha. His grand and often cruel plans for the ninja world would be further explored in a darker fanfiction.
Captain America is very likely to be featured. He has that paternal, moral - compass kind of vibe. He would be the one to gently tickle the 'daughter' as a way of showing his soft side. Thor could also be a popular choice. His big - hearted and sometimes child - like nature would make for some great tickle - related antics with the 'daughter' figure.
Steven is the core, and among the Gems, not only the Crystal Gems but also Bismuth could potentially be part of it. Bismuth has a strong personality and her own connection to Steven. She could bring a different kind of energy to the harem. Also, Connie could be involved in some way, perhaps as a rival or a friend who helps Steven navigate his relationships with the Gems. Her human perspective could add an interesting layer to the story.
Thor would be one of the active ones. He has a lot to share from his Asgardian experiences. Hulk, well, he might not type much but when he does, it would be in short, powerful statements. And Scarlet Witch could be in there too. She could use her powers to sense if there's any danger approaching the chatroom or the Avengers themselves while chatting with the others.
Doctor Octopus is also quite popular in 'no way home fanfic'. His complex character and his unique abilities make him a fascinating subject for fanfic writers. They can explore his backstory more or create new scenarios for him in the 'No Way Home' universe.
The Marker in Dead Space is also likely to be a key element in a crossover fanfic. It has such a powerful and mysterious presence in the Dead Space universe. In a crossover with a horror - mystery series, the Marker could be the source of all the strange happenings, and characters from both universes would have to figure out how to deal with it or stop its influence.
Natsu would be a likely candidate. He's always eager for a fight and his reactions to the intense battles in Death Battle would be interesting.
Dante is a very likely character. His flamboyant and confident personality would make for an interesting contrast in the MLP world. He could be the one who brings a different kind of action and chaos to the usually peaceful pony lands.