Red vs. Blue fics all seem to be about a hitherto unknown Freelancer, someone new joining the Blood Gulch crew, or a hitherto unknown AI. Sometimes all three at once.
One of the most common fanfics is "Carlos takes Cecil to meet his family". Expect varying levels of culture shock, homophobia and fluff.
A popular fic concept is that Cecil isn't human in some way, and is terrified of how Carlos will react. Carlos, for his part, is totally fine with it. The smut is optional; the fluff is not.
Ever since the Crypton Vocaloids as a big family started cropping up in officially licensed works, other studios' voicebanks being grouped into family units has grown into a popular topic.
Vocaloids being presented as living programs or actual singing androids on the market, and their new lives with their master.
Gumi trying and failing to get into a loving relationship.
Wattpad is known across the internet as a website full of specific plots that appear in hundreds of users' fanfictions, whether it be getting kidnapped by your crush, being sold to a handsome gang leader by your poor parents, or a retelling of the author's favourite movie with themselves and their celebrity crush. Strangely, they all seem to star bad boys and/or boy bands. Also, they all begin with the main character (always a girl) getting out of bed to look at herself in the mirror and think that she isn't pretty enough to have friends/a boyfriend.
After the show's Series Fauxnale, Eddsworld fics in which Tord returns a second time have become very common. He'll usually be back either to take revenge on the others for destroying his giant robot and ruining his world domination plans, or because he's remorseful for attacking them and wants to make amends. More often than not however, it's a mix of both, with Tord returning for revenge, getting defeated and undergoing a Heel–Face Turn with Edd, Matt and (almost always begrudgingly) Tom's help.
A common crossover plot for Hazbin Hotel involves a morally grey character from the other half of the crossover dying, arriving in Hell, and moving into the hotel.
There's loads for "The ReviewMust Go On", whether it's Donnie's friends refusing to let him go or trying to explain why Critic had suddenly turned evil.
It’s common for SCP Foundation stories, especially on Archive of Our Own, to have Dr. Clef go through angst relating to his trauma from the events depicted in SCP-4231 before finding comfort in another character. Said other character is usually Dr. Kondraki, SCP-166, or an Original Character.
