I have the known lands in my mind, but if you want to do a supplement, you have to start somewhere. Best place to do a supplement/adventure for your world is do Bottom Up! In formulating a Rolemaster campaign (where I have one player) in the Forgotten Realms I came across SHADOWDALE -- the campaign setting for FR (it's a campaign setting within a campaign setting).
What a wonderful idea!
Everything not withstanding, to write a supplement that contains RM stats to start with the Known Lands it is best to start at the bottom and work your way up to meet your Top Down Design. So, I am thinking of a village that is in a neutral zone and is governed by the three powers of the Caithness Islands: Caithness, Massalia, and the Horde.
Three things so far about this village:
a. It's about the size of one of the Dales in the Forgotten Realms.
b. The three powers have three governable regions, cutting it into threes except for the "square." The Massalian Elves have one region, the orcs have another, and the humans of Caithness has another.
c. It's cultivated, but is surrounded by a thick forest (about as thick as Sherwood, or maybe thicker).
I'll have to think on it to decide some more things come up. but I do believe I have the seeds for a supplement that can be published through the Guild Companion.