I think Groot from Guardians of the Galaxy is already quite an unexpected character in the Infinity War story. But if we want to be even more out - there, how about Snoopy from the Peanuts? Imagine Snoopy flying his doghouse into the middle of the battle against Thanos and using his imagination - powered antics to distract the villains. It would be a really wacky and unexpected addition.
Wall - E could be an unexpected character. He could use his resourcefulness and ability to survive in harsh conditions to help the Avengers. Maybe he could find a way to hack into Thanos' technology or use his compacted trash cubes to build defenses. His innocent and curious nature would contrast with the dark and serious nature of the Infinity War plot.
The Genie from Aladdin would be a very unexpected character. He could use his magic to help the Avengers in unexpected ways. For example, he could grant wishes that would turn the tide of the battle. He could wish for Thanos' gauntlet to disappear or for all the fallen heroes to be brought back to life instantly. His comical nature would also add a touch of humor to the otherwise serious Infinity War setting.
Mario. The plumber from the Mushroom Kingdom in a crossover with Final Fight would be quite a shock. However, his acrobatic jumps and ability to use power - ups could add a unique element to the fanfiction. He could jump on the heads of the Final Fight thugs or use a fire flower to defeat them in an unorthodox way.
It could be Lincoln. He's shown to be resourceful in the Loud House. In the context of the Infinity War, he might be able to adapt quickly and find ways to use the power of the Infinity Stones against Thanos if he got his hands on them.
Wall - E could be a very unexpected character. He's from a post - apocalyptic world where he's all alone collecting trash. In an Ever After High crossover, he could land in the school's garden and start cleaning up. The students would be so confused by this little robot. But he could also teach them about the importance of taking care of the environment, which is something not really focused on in the fairy - tale world of Ever After High.
The Doctor from Doctor Who. He could bring his time - traveling TARDIS into the Justice League's universe. His wits and knowledge of different times and places could be a huge asset in solving a mystery that the Justice League is facing, and his alien nature would add an interesting element.
I think Groot from Guardians of the Galaxy would be really unexpected. Imagine a Hogwarts teacher trying to communicate with a talking tree. It would be hilarious and full of misunderstandings at first. But then they could find a way to work together, maybe Groot could use his strength to help with some physical tasks around Hogwarts in a really unique way.
Gandalf from 'The Lord of the Rings'. His magic and wisdom could add a new dimension to the story. He could help the X - Men in their fight against Apocalypse, perhaps using his magic to counter some of Apocalypse's powers or providing strategic advice.
I think Gizmo from the Teen Titans could be really unexpected. He's used to dealing with high - tech and more colorful villains. Throwing him into Arkham Asylum, with all its dark and gritty atmosphere and dangerous inmates, would create a really interesting dynamic. He could try to use his gadgets to either escape or try to take over the asylum, which would be hilarious considering the much more menacing villains there.
Probably Genie from Aladdin. Genie is all about granting wishes and his larger - than - life personality. In a Frozen crossover, he could offer to grant Elsa a wish, like making her ice powers more controllable or finding her a true love in a really over - the - top way. It's not a combination you'd think of right away.
Batman would be a great character to focus on. He doesn't have superpowers like the others but his intelligence and strategic mind are his greatest assets. In a 'Justice League Infinity War' fanfic, he would be the one trying to figure out the complex nature of the Infinity Stones, coming up with plans to outwit Thanos. He might also have some tense moments where he has to rely on the super - powered members of the League in ways he's not used to, which could lead to some great character growth and interesting interactions.
I think Riku and Captain America would be a great pair. Riku has a strong sense of justice and has fought his own inner demons. Captain America is all about truth, justice, and the American way. They could bond over their shared values. Riku could show Captain America some of his cool Keyblade moves, and Captain America could teach Riku about strategy and leadership in battle.