Just trying to lump this together in case that other thread disappears:
There has to be a drive that is shared but not the content. The content can be sorted user end as they see fit.
Even saying that "Its blue dragon month! Everybody do a blue dragon theme!" its a nightmare. That just sounds intimidating. Some people play D&D and never even see a dragon much less a blue dragon just by coincidence. Then there is the idea of replication and shameless "you all get a star". It has to be the opposite.
I was thinking for a universal sort of shared idea of a war of the wizards that is a concept that people can identify with and everyone's wizard of their world is trying to destroy each other. Not directly, but you can mention the other wizards from time to time with a notice. Then anyone can join in and have their world in the background and perhaps showcase a new element.
I was thinking of an ongoing theme of a(n) World "Astral" War II that never ends with antagonizing wizards then its like a Lovecraft/Howard thing where its separate completely but tied in through the brief mention of the animosity if not specifics.
Ok, what is your planar territory name of choice that you are going to creatively develop?
A wizard who is trying to destroy other wizards from other worlds but can/will never directly affect anything specific, of course, and instead its a red shirt wizard from an alternate world of your own who can be destroyed/interacted and everything else can be handled in an "Unaussprechlichen Kulten" manner where they can't be touched or interacted with without the direct consent or cooperation but can be mentioned in a non-interacting way.
Then this gets placed where ever. For me it would be my blog, of course, then implementing it into different forms now and then. Perhaps as small as an intro quote that segues into something else or as large as a planar exploration piece.
I think this is everything. So now its all compact.
"Damn that Weigelius of the Skull Dimension!" the wizard Mystagian exclaimed. "What is it, master?" Mystagian's familiar, a dog-like creature with bat wings, showed concern and padded over. "He tricked me! I thought this was a Helm of Comprehending Languages and Reading Magic. But no! It's a Helm of Constipated Thoughts!" "Is that bad, master?" the servile familiar asked. "Of course it's bad! It means I can't complete any reasonable thoughts, or concentrate long enough to re-memorize my spells. Soon, I'll be at the mercy of not only Weigelius but all of the other wizards as well. And then... And then... Uh, what was I saying?" "Something about the other wizards, master. But I didn't understand it all."