In a fanfiction scenario, Harry might be able to transfer some of his own magic related to the Master of Death to Tsuna. This could give Tsuna new abilities like being able to see the souls of the dead, which could be useful in both detecting enemies and getting advice from past heroes in both their worlds. This combination of abilities would make them a very formidable team in any adventure or battle they face.
Tsuna's ability to ignite his Dying Will Flame could be combined with Harry's knowledge of the magic related to death. Maybe the Dying Will Flame has some connection to the concept of death in a way that Harry, as the Master of Death, can understand. He could then help Tsuna control and intensify his flames, perhaps by using some death - related incantations or rituals that he has learned in his own world.
Well, Harry's magic is more traditional in the sense of spells and potions from the wizarding world. Tsuna has his own unique abilities in his world. When combined, Harry could use his spell - casting skills to enhance Tsuna's attacks. For example, Harry could cast a spell on Tsuna's gloves to make them more powerful when he uses his X - Burner attack.