You can try fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. These platforms have a vast collection of fanfictions from various fandoms, including those related to 'Once upon a time' and 'Watch the future'. Just use their search functions to look for the specific fanfictions you're interested in.
One common theme is time -travel. Since both shows deal with different timelines in some way, fanfictions often explore more about time -travel and how it affects the characters. For example, a character might go back in time to change a crucial event.
I'm not sure specifically as it's not a widely known common story. But generally, it might be about a badger who has some sort of important role in a past and future context. Maybe the badger was a great leader in the past and is prophesied to be one again in the future.
Maybe there is a group of characters. One could be a brave warrior, another a clever strategist, and there could also be a mysterious figure whose identity is slowly uncovered throughout the story. Together, they form a team that has to deal with the challenges presented in the 'once and future graphic novel'.
The themes include the cycle of violence and the attempt to break it. Arthur tries to create a new order with the Knights of the Round Table to end the feudal and violent ways of the past. And the theme of love is there too, in different forms - love between friends, like Arthur and Merlin, and romantic love which also affects the characters' actions and decisions.
King Arthur is obviously a very important character. He is the central figure around whom the story revolves. Merlin is also crucial as he is Arthur's mentor, guiding him with his magic and wisdom. Then there are the Knights of the Round Table like Lancelot, who is known for his great skills in combat and his complex relationship with Arthur and Queen Guinevere.