Popular tropes in their fanfiction include the 'rivalry - turned - friendship - or - more' trope. They might start off as rivals, competing in some aspect like who can solve a mystery faster or who is a better fighter. As the story progresses, this rivalry turns into a strong friendship or a romantic relationship. The 'in - over - their - heads' trope is also seen often. They get involved in a situation that is far more dangerous or complex than they initially thought, and they have to rely on each other to get out of it.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can simply search for 'Cassi Sandsmark and Tim Drake' in the search bar. Another option is FanFiction.net, though the content there might vary in quality. Some Tumblr blogs also share and recommend good fanfictions related to them.
I really like 'Tim and Cassie: Growing Bonds'. In this fanfic, it starts with them having a bit of a misunderstanding which is really relatable. Then as the story progresses, they start to understand each other better and their friendship evolves into something more. It has some really great dialogue that makes the characters come alive.
The 'alternate self' trope is also common. He could meet a version of himself that is evil or has a completely different life path. This could lead to self - reflection as he sees what he could have become. For instance, an alternate Tim might be a member of a criminal syndicate. Tim would then have to decide whether to try to redeem this version of himself or just focus on getting back to his own dimension.
I'm not sure specifically about 'lemon' fanfics as they often involve inappropriate or adult - themed content which is not suitable. However, there are many great fanfics about Tim Drake and Cassie Sandsmark that focus on their relationship in a more appropriate way. You can try searching on fanfic platforms like Archive of Our Own and filter for the Teen and up rating.
Another option is Tumblr. There are often fanfic recs (recommendations) on Tumblr. Some blogs are dedicated to sharing great fanfictions, and you might be able to find some good Tim Drake and Cassie Sandsmark fanfictions through those blogs. Just search for relevant tags like #TimDrake #CassieSandsmark #Fanfiction.
One popular theme could be Tim Drake's dreams and how they might relate to his crime - fighting life. Maybe in his sleep, he has visions of future cases or battles, and his subconscious is working on solving problems. Another theme could be his rest and relaxation, showing a more vulnerable side of him as he sleeps, away from the action and danger.
One popular trope could be the 'friends - to - lovers' trope. Tim Drake starts off as friends with someone and then gradually they realize their deeper feelings for each other. It's a sweet and relatable trope that can be really well - done in fanfics.
It's popular because Tim Drake is a beloved character. When he's portrayed as neglected, it adds an interesting layer of depth. Fans like to see how he overcomes this situation.
Archive of Our Own is a great place to start. It has a wide range of fanfics, and you can use the rating and filtering system to exclude any adult - themed stories. You can search specifically for Tim Drake and Cassie Sandsmark, and then select the Teen and up or General Audiences ratings.
Some great Tim Drake centric fanfictions include those that explore his relationships. For example, there are fanfictions about his friendship with Stephanie Brown. These stories show how they support each other through various challenges. Also, there are ones that look at his mentorship under Batman, highlighting how he grows as a hero with Batman's guidance. Additionally, fanfictions that put him in crossovers with other heroes like the Teen Titans can be really interesting as it shows his ability to adapt to different hero teams.