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.
Erza is another strong possibility. Given her strong sense of justice and combat skills, she would likely have a lot to say about the battles she watches. She might analyze the fighting styles and think about how she could incorporate some of the techniques into her own battles. Gray could also be prominently featured. His cool - headedness (most of the time) would make his reactions to the chaos of Death Battle unique. He might compare his ice - make magic to the powers he sees in the battles and wonder how he could improve or if his magic is strong enough in comparison.
Another character that might be featured is Jellal. His past and his powerful magic make him an interesting character for death battles. He could be pitted against both Fairy Tail characters and characters from other series in the fanfiction. Plus, characters like Mirajane in her Satan Soul form also have a high chance of being included due to their unique powers and abilities.
Obviously, the main characters from the 'Fairy Tail' anime such as Natsu, Lucy, Gray, and Erza are likely to be the main characters in this fanfiction as well. Since it's about them watching the anime, they would be at the center of the story.
Erza is also very popular. Her wide array of armors and combat skills provide a lot of material for death battle scenarios. Writers can come up with creative ways for her to use her different armors in battles against various foes.
Obviously, Natsu would be one of the main characters since the fanfiction is centered around him fighting. His friends from Fairy Tail like Lucy, Gray, and Erza are also very likely to be main characters. They would be the ones watching Natsu fight and having reactions to his actions in the battle.
Well, in this 'fairy tail watches the past fanfiction', you can't really leave out the core group. Natsu is a must - his hot - headedness in the past compared to his growth in the present would be a great focus. Lucy, with her celestial spirit magic, could have a unique perspective on the past as she delved deeper into the guild's history. Gray, with his ice - make magic and his past traumas, would be an important character to explore in this context. Erza, the strong and fearless female knight - like character, would surely have a lot to learn from her past self. And Happy would add some light - heartedness to the whole situation as they all watch the past.
Gray is another candidate. Gray's cool and collected demeanor could contrast with Tobi's more mysterious nature. They could get into arguments about the best way to handle a situation, like when facing a powerful enemy. Or, they could find common ground in their skills and work together to protect Fairy Tail from danger.
Perhaps the fanfiction explores how the Fairy Tail worldviews on magic compare to the diverse combat styles shown in Death Battle. It could lead to some characters re - evaluating their own magic abilities. Another aspect could be the crossovers that might occur within the fanfiction. For example, characters from Fairy Tail might get involved in battles they otherwise wouldn't in their own universe, which could create new alliances or rivalries.
Another character who could run a burger joint is Macao. He seems like the type who would enjoy the hustle and bustle of running a business. He has a lot of experience in the guild and is good at dealing with people. He could use his fire magic to cook the burgers in a unique and fast way, adding a special touch to the food at his burger joint.
Lucy is also often featured. With her celestial spirit magic, she can call upon different spirits to aid in battles, adding an interesting strategic element. Plus, her character development over the series makes her a great focus.
Natsu would be a strong contender. His dragon slayer magic gives him great strength and endurance, and his determination to win would be unwavering.