One possible source of angst in such a fanfic could be the internal struggle of Majin Vegeta. He has this new, dark power that might put his relationship with Bulma at risk. For example, his altered state of mind due to the Majin influence could lead to him pushing Bulma away, fearing he'll harm her.
The theme of lost dreams can also be found. Maybe Changkyun's character had certain dreams that he couldn't achieve due to various circumstances. This sense of loss and disappointment is a great source of angst in these fanfics. For example, he might have dreamed of a normal life but is trapped in the world of fame.
Perhaps when Bulma tries to get through to Vegeta but he snaps at her out of his newfound aggression. This is a really angsty moment as it shows the breakdown of their communication.
You can start by searching on popular fanfic platforms like Archive of Our Own. Use the tags 'Suga x Reader' and 'angst' to narrow down your search. There are usually a lot of great options there.
There's also 'Fading Memories'. In this fanfic, Changkyun is dealing with the loss of a loved one. The angst is palpable as he tries to hold on to the memories while also trying to accept the reality of the situation. It's a very moving piece that will tug at your heartstrings.
Friendship is a very common theme. Since they are both characters from Total Drama, many fanfics explore how their friendship started and developed over time. Another theme is rivalry, as they sometimes compete in the show and fanfics can play with that aspect.
One common theme is friendship. Their relationship often starts as an unlikely friendship that blossoms over time. They learn to understand and support each other.
One common theme is loyalty. Noah and Mokuba often show deep loyalty to each other in these fics. For example, in many stories, Noah will go to great lengths to protect Mokuba from harm.
Adventure is also a popular theme. Writers often take the two characters on exciting quests or missions. This allows for a lot of action - packed scenes and the development of their relationship in the face of danger.