The Legend of Korra comics are definitely worth a read. They capture the essence of the franchise and bring fresh perspectives. The quality of the storytelling and the visuals make them a great addition to the series.
The 'lemon' aspect might be a bit hard to come by in a clean way as it often involves inappropriate content. But for normal Korra and Mako fanfictions, you could also try FanFiction.net. Some stories there might show their relationship developing, their adventures together, and how they support each other, which can be really interesting to read.
Yes, there are many. Some focus on crossovers between the two universes, like characters from RWBY meeting those from Legend of Korra. You can find them on fanfiction websites. Just search for the relevant tags.
One good experience could be finding fanfictions that expand on the relationships between the characters. For example, those that explore the friendship between Korra and Asami in more depth. It gives a new perspective on their bond.
An OC could be a descendant of a long - forgotten Avatar. This OC might start having strange dreams or visions that lead them to discover their heritage and their potential power. They then team up with Korra to learn how to control this new - found power and use it for good. Or perhaps an OC who is a master of a new form of bending that combines two existing bending arts in a unique way. For example, a combination of water and earth bending to create a sort of'mud - slinging' technique that can be used both defensively and offensively. Another option is an OC who is a mechanic in Republic City but discovers a way to use their mechanical skills to enhance bending abilities, like creating a device that helps fire - benders focus their heat more precisely.
First, create a unique backstory for your male OC. For example, if he's a waterbender, think about how he learned his skills. Maybe he was taught by an old master in a secluded tribe. Second, give him distinct personality traits. He could be a bit of a rebel but with a heart of gold. Third, tie his story into the existing Legend of Korra world. He could be involved in the political unrest or the fight against new villains. This will make the story feel more connected to the source material.