So Conan abdicates his throne, sails across the Western Ocean and is never seen again. I’m thinking of starting a campaign that takes place shortly after this. My Lore knowledge isn’t very deep, so is there any sources of lore for the world around this time period. I know Conan’s son is King of Aquilonia. But who else is probably still alive, who is probably dead. What are the geopolitics? Who are the big threats. I know I could just make up something up, but I’d like to plum some ideas from the actual lore if possible.
There is no fast answer to what you are asking, and in your case I would recommend to do some homework before you plan accordingly. Nothing prevents you to do what you want, you are the GM and the game is yours, but to make it plausible and to provide proper immersion for your players (especially if they are familiar with the Conan books), you should read.
Good starting point is https://hyboria.xoth.net/history/hyborian_age.htm, followed by check at the Gazetteer for the location where you will situate the game https://hyboria.xoth.net/gazetteer/index.htm. Afterwards I would consult the old Mongoose books for more lore or check if the current books have sufficient information. For example the Messantia sourcebook from Conan d20 was much much better than what we have in the 2d20 sourcebooks. Took all the guild and noble family information from there for my campaign to provide truly immersive and working world.
If I remember REH’s hort article about the Hyborian Age correctly Conan was the last great ruler of Aquilonia and after him the Picts begin to invade and the Age eventually ends with a great flood.
“But it was in the west that a power was growing destined to throw down
the kings of Aquilonia from their high places. In the north there was
incessant bickering along the Cimmerian borders between the black-haired
warriors and the Nordheimir; and the AEsir, between wars with
the Vanir, assailed Hyperborea and pushed back the frontier,
destroying city after city. The Cimmerians also fought the Picts and
Bossonians impartially, and several times raided into Aquilonia
itself, but their wars were less invasions than mere plundering
forays…”