Oh boy! I actually really love writing canon characters, althougg I do tend to gravitate towards minor characters. Minor and side characters have the flexibility of OCs, but also the appeal of being established in the universe already. They tend to feel more real to me in fandom games, especially if canon characters are kicking around.

I like using the framework provided for canons to explore characters that otherwise wouldn't have as much development. I think the awesome part of writing canons is the challenge in writing them believably [I]and[/i] finding your own voice in them. Canon characters are sometimes less flexible than OCs, but there's also groundwork already laid down for you. That's really helpful in fleshing out believable and engaging characters. Minor canons also seem less prone to blatant self-insert and Mary Sue/Gary Stu traits than typical OCs.

There's also the appeal of taking a minor character that received [I]awful[/i] characterization in their canon and exploring other alternatives. Some of my best games have been started because of "this writer has no idea how to write women, and definitely not teenaged girls, I will fix this" conversations with fellow writers.

In that vein, AUs are super fun and refreshing. They're massively popular--look at the mountain of fanfiction and roleplays for coffee shop, florist/tattoo shop, fantasy, modern slice of life, and other AUs in just about every fandom ever these days. Canon characters provide the common ground for these works. Since everyone has a strong idea of who these characters are, it's easy to thrust them into new settings and explore new facets of their personalities. You get all the joy of original settings and new lore with established and well rounded characters. That's the best of both worlds!

Obviously not everyone agrees on characterization, but people bicker about how characters behave within the canon itself, so it's no big deal if someone thinks your portrayal of Hermione Granger is garbage or not. It's all for fun anyways.

I've been writing canons and OCs alike over the years, and I love both types of characters. I think the mindset of canon as limiting is really common, but I don't really agree with it. It's certainly a little intimidating, but it's just as fun and rewarding as writing OCs.