I too am jumping on the #Dungeon23 bandwagon. It seems like a fun and manageable way to do a large project, and I hope that having so many others do it simultaneously I'll stay motivated for longer.
Though I don't want to plan too far ahead I do want to settle on a strong theme, some setting, something of a vague history of the dungeon, and collect a bunch of ideas I think would be fun. For this I am stealing the slush pile format, which I learned about through Dan of the throne of salt blog, but has been used to great effect by others as well.