1. Random Encounter Tables for areas all over the Flanaess.
I can see these a general encounters for specific areas and regions and left to the DM to detail. Detailed encounters that add some character and specifics to each encounter. Connected encounters that happen in sequence. Branching encounters that happen as players make different choices.
Connected encounters might happen over a period of hours, days or even weeks. For example in the Night Below/TOEE encounters I have been working on the Players encounter a party of goblins or people or places attacked by goblins. As time goes by the goblin encounters increase in severity and this alters many future encounters as this invasion of goblins affects more and more of the area they are in. As another example the players encounter a thief in the city of Greyhawk. Hours later they see the same thief and react. The thief is killed, caught or escapes, hours after this a group of thieves attacks the players in revenge.
With a Branching Encounter the initial encounter will have different results. for example the Greyhawk thief is encountered. If they are killed it triggers one branch of encounters, if they are caught a different branch, if they escape, a third branch of encounters is triggered. This much as is done in a choose the path adventure book but from a random table of possibilities rather than a single flow of one encounter to another.
The more encounters created, the better.
2. A random building/shop generator table for a city. We create a list of possible shops or buildings. Then a list of detailed buildings. For exmaple.
c. General Goods
e. Gambling Hall
1. The Acorns
2. Ancient King
3. Bates Complex
3. A listing of goods and prices for regions, cities and areas with what is produced and traded. I can this as having great use as background detail for DMs and a way to generate potential adventures.
Those are 3 ideas. Happy to hear others and I will list more.