I'd recommend not putting too much time into it. There's little point celebrating a twenty year anniversary of writing if you spent 10% of that time working on some anniversary project. By all means invite people over. Have a party. Get drunk and play ping pong. But don't be one of those people who spends more time "being a writer" than they spend writing. Put your effort into writing the things you really want to write.
I can understand that too but you have music artists that release special albums that were released back 20 years ago so the idea of it is not uncommon. I just wanted to do different. Now it might not be special or rare compared to others but this is about my own writings and that my own characters have never mixed with each other.
I would it to be a boring year if I just did the same stuff as I would normally do. My main issue as a writer is that I think outside of the box alot and most people don't "get me". Most writers follow the boring pathway, write a book, try and get it published, repeat.
I'm still not 100% clear on what you're hoping to do (neither of those examples sound special/rare, in that plenty of other people have done them), but going back to my original point: it seems kind of silly to spend any significant amount of time working on something to celebrate a 20th writing anniversary. It strikes me as being a little like a professional athlete lazing around in front of the TV for a week to celebrate 20 years of being an athlete. Time spent working on the anniversary project is time not spent working on your actual story/stories.
It also seems like a bit of a Catch 22. If you'd write the thing anyway, it's not special. If you wouldn't write it anyway, you're letting yourself get distracted, surely?