The rise of Set in City-State is the common thread across 20 years of campaigns.
This needs further explanation.
The hallmark of my early Wilderlands campaigns was "Players have an impact." I kept track of what players did, both major and minor, and used that as background for the next campaign. Most of my players got what I was doing and liked it.
However the mid 80s was the time of Dragonlance, time of me entering college, Watchmen, etc. Our hobby was becoming more sophisticated in how it approached plots and stuff. But the most important influence was me understanding history, culture, religion and perceiving how they interconnected.
There was a lot of good adventuring ideas to found. Intrigues, plots, and organizations that I can use to spice up my game and give more depth. What didn't want to do is run Dragonlance. By that I mean the railroaded plot found in the Dragonlance modules.
I ran Dragonlance, made an honest stab at trying to run a fun game. Except it wasn't that fun. This was my second college campaign and the players were soon asking to go back to the Wilderlands. That third campaign was the last campaign of AD&D I ever ran and one of the most fun. It resulted in some of the best role-playing I ever seen. Along with the fall of the Viridstan Empire when the paladin Endless Star killed the last Viridian Emperor on his throne.
Along with this I ran an equally fun campaign back home where Lord Divolic (Trained Killer), and Count Travlin (Boris the Bagger) entered the Wilderlands. This was my first "evil" campaign where Lord Divolic was a Myrmidon of Set. I always disliked the concept of a CE anti-paladin. To me the anti-paladin was LE sworn to uphold the tyrannical cause of their god as the LG paladin was sworn to uphold the good cause of his god. The CE gods had better things to do with their power rather than entrust it in maniacs with potently to become a rival.
By the end of the campaign Lord Divolic was Warden of the Southern March in service to the Overlord of City-State with Travlin owning extensive estates in the March. The wealth behind their power was gained when they literally ripped the Tomb of Horrors out of the earth.
The phrase "Rob! Can you read that again and more slowly?" is forever burned in my mind as I realized with horror of how much wealth I just gave these two characters. Let just say that the true wealth of the Tomb is not in the loose treasure lying on the floor. Those of you with copies of the original see if you want find what I am talking about. Note the Wizard remake for 3.X fixes this issue you have to look for it in the original.
Between 1985 and 1988 my Majestic Wilderlands underwent a drastic revision. I expanded the scale, redrew the maps, wrote down the half dozen timelines covering over 6,000 years of history. I had many of the same names, a few shared events but the Majestic Wilderlands had ceased becoming Bob Bledsaw's Wilderlands.
I decided to rationalize my older campaigns. Luckily my original notion of allowing players to have an impact made this easy to do. This because all my original campaign started with a handful of premises to get them started and from there it was me reacting to the player and the player reacting to me. I used modules that fit the locales they were going or what they were searching for.
Since I had one really fun "evil" campaign that revolved around Set, I decided Set was going to be the main antagonist. But I also had a bunch of other campaigns that had nothing to do with Set. So while the Rise of Set was the main idea there was other things that were going on too that were only related because they were in roughly the same locale and same time.
I took my box of notes (which I still have today) organized it and figured out my history up to the present day. And then I stopped. Because the rule of players being able to make their mark on my Wilderlands was still important. Instead I would record what happened the previous campaign. Add it to the background. And when a new campaign started I would jump the new campaign a year or so and say "OK this is what is happening now."
Over the last 20 years my Majestic Wilderlands has jumped mostly in two year increments. When I first used the new Majestic Wilderland the year was 4438 as the last AD&D game I ran ended in 4437. Then next series of campaigns (home and college) was 4440, then 4442, and so on up to the present of 4452. There were some bigger jumps for some of the longer campaigns, but most campaigns lasted in-game for a year or two.