This is a fun list! Off the top of my head, here's other potential additions:

- make and wear a cosplay

- engage in roleplay

- participate in a zine or other fandom-related group project

- get involved in pointless fandom drama (don't do this, it's not worth getting that 100%)

- read every single fanfic that's ever been written for the game/book/show/etc (may or may not take a hundred years. More achievable version would be "read everything that sounds interesting to you")

The more I think about this concept, the more I think that a completely literal 100%-ing might be hazardous to your health lmao. Maybe I should think of it as getting 100% all worthwhile achievements instead of doing 100% completion of all possible fandom-related actions.