Does anyone else play their character as an existing character from a different franchise (Example: playing a route where your character is basically Conner Kent/Percy Jackson/Alice Johnson/etcetera)? If you do:
Sometimes I do this. For example my main MC for Samurai of Hyuga is based on a character in the Legend of the Five Rings book series, Hitomi was the protagonist of the sixth book in the series, and my MC for Wayhaven Chronicles is also based on a book series protagonist (won’t name this book series since this one is written by a hungarian author so I suppose that would be less known here).
Basically I watch out that I keep the personality of my MC as close as possible to the characters I make them based off. As for OOC choices: mostly I keep this to the romance area (since most already existing characters are hetero), but other than that I try to pick the choice options I think the borrowed character would make.
Sometimes I find that just as fun as creating my own character tho it’s really rare that I like a character that much and the game setting also has to have a similar vibe to the book or whatever I borrow the character from.
Haven’t quite done this on IFs, but when playing Long Live the Queen I’ve done playthroughs of Irene and Helen from the Queen’s Thief just because.
I have had a couple of people mention they were doing this with The Parenting Simulator, raising their kid to be similar to someone from a story or anime. It definitely works best with the most blank slate protagonists.
Role playing as an existing character? Never tried that before, but why not? Maybe on a replay just to keep things fresh, and my choices varied without feeling less organic than I prefer them to. 
I’m considering to roleplay a male characters genderbent version. Now I just gotta find a male character whom I like enough and a game in which he would fit.
Sometimes I’ll roleplay MCs based on existing characters, or at least their name and some traits, although it can be difficult when there are no options that character would pick. I usually just consider those decisions that deviate from canon behavior to be like how the same character in alternate universe stories can be slightly different.