Magic and mystery are likely main themes. Merlin is a well - known figure associated with magic, so the novel probably explores his magical powers and the mysterious world he inhabits.
They could have adventures where they explore ancient magical places. Maybe they find a long - lost magical artifact that has the power to change the fate of Camelot. Merlin, with his knowledge, and Aithusa, with its dragon abilities, would be a formidable team in such quests.
Morgana might also be involved. If she were to travel through time, she could either try to change the past to gain more power in the present or she could be sent to the future to see the consequences of her actions. This might lead to a change of heart for her, or it could make her more determined to achieve her goals depending on what she witnesses.
They could have an adventure where they help Camelot defend against a dark sorcerer. Harry's combat magic from Hogwarts would be useful, and Merlin's knowledge of Camelot's magic and its secrets would be crucial. Another adventure might be to find a lost magical artifact that is important for both Camelot and the wizarding world. Along the way, they might encounter magical creatures unique to Camelot that Harry has never seen before, like the Questing Beast.
They could go on quests to find ancient family artifacts. Maybe there's a magical sword or a powerful amulet that has been in their family for generations. They would set out to retrieve it to protect their family's honor or to use it to save their kingdom.
If Merlin died in the fanfiction, it could trigger a series of tragic events. The balance of power could tip, and the villains might seize the opportunity to attack. It could also bring out hidden strengths in the remaining characters as they try to cope with the loss.
Perhaps some of the snooty nobles who look down on Merlin and constantly try to get him into trouble. They could be making fun of his clumsy ways one too many times and he just snaps and hits them with a shoe. It would be a moment of satisfaction for the readers who don't like those arrogant characters.
One possible secret could be that Merlin is a powerful sorcerer from an ancient and powerful magical lineage. Maybe his family has a long - standing connection to Camelot that has been kept hidden.