One common theme is teamwork. Given that all three characters are powerful in their own universes, fanfictions often explore how they work together to fight a common enemy. For example, they might team up against an alien threat that is too powerful for any one of them to handle alone.
Well, another place could be some dedicated superhero or Dragon Ball fan forums. Sometimes fans share their own fanfictions there. For example, the Dragon Ball Z subreddit might have threads related to Gohan crossovers, and you could potentially find links to fanfictions involving Starfire and Supergirl too. It might take a bit more digging though compared to the big fanfiction sites.
The winners in Injustice comics aren't always the same. It often depends on the circumstances, the alliances formed, and the strategies employed by the characters.
Robin could be a great addition. He could act as a bridge between Starfire's more light - hearted nature and Batman's seriousness. His acrobatic skills and detective training could complement both of them in a fight. Another good character would be Nightwing. He has a connection to Batman already and his charm could add an interesting element when interacting with Starfire.
Beast Boy is another good option. His ability to transform into different animals can bring humor to the fanfiction. He could also be a mediator between Starfire and Blackfire, using his light - hearted nature to ease the tension between the two sisters. His friendship with Starfire can also be explored further in relation to Blackfire.
I think it would be Flash. Flash is friendly and open - minded. He would be intrigued by Starfire's powers and her alien origin. He would likely be the first to approach her and start a conversation. They could bond over their quick - thinking abilities during battles, as Starfire is also very adaptable in combat situations.