I'm in a group that runs multiple Pathfinder AP's depending on who has time to GM. I love plotting in crossovers between these AP's. An NPC who's disappeared in our Kingmaker campaign, reemerged in our Skull & Shackles campaign, which is set two years in the future from the Kingmaker campaign's timeline, for instance.
Time has come for me to GM Rise of the Runelords. As I've understood, Jade Regent is something of a sequel to Rise of the Runelords, and it would be fun to follow up with it after. Now, for the question..
Question: Seeing as Rise of the Runelords is the first AP of pathfinder, I wanted to have it take place further back in time, ca. the year 4688, as opposed to 4707. Would this be a problem, regarding continuity and events that transpire on Golarion?
(Pardon my hyped rambling, the following isn't necessary to read..)
The reason I want to set it back in time, is to evoke more of a primal feel. Have it really feel like the one that started it all. Thus I will avoid some of the more exotic elements, such as gunpowder and stick to concepts that seems somewhat familiar within core lore. Of course, with the pulp fantasy nature of pathfinder and the somewhat urban setting of Varisia, high fantasy is far from possible. In stead I want to try and invoke something of a cult B-movie feel, utilizing humor (Goblins), grandiose nightmarish horror (Lamashtu worship) and tongue in cheek smut (possible NPC relations) side by side, which it seems like RoTR very well can be fitted to. Planning on doing an 80ies movie binge, before I start planning. Though I'm digressing..
I might de-age some of the characters to try to adjust the timelines a bit, should it be an issue, such as e.g Kendra Deverin being a young aspiring mayor for instance.
Some random thoughts. Of uncertain helpfulness...
Not sure how moving the start time back 19 years makes it more primal. The outcomes you describe seem just as doable at the as-written calendar, since many of them will be driven by GM restriction, etc.
Shattered Star is, I think, technically the sequel to RotRL with Jade Regent following. Also of note - they've announced Return of the Runelords as an AP (not sure on schedule.) Altering dates for Runelords will likely drive possible impacts for all of those AP's. It seems your group likes a shared timeline so that might have consequences should those other AP's enter the rotation (if they haven't already.)
That said, there should not be too many consequences to the timeline of the AP itself. You'll just have to watch for various elements - there are events that are pretty critical that happen in Sandpoint 5 years before the AP starts - those will need to be moved. And if you go backwards in time you move closer to the origins of Sandpoint, Magnimar and other places like Turtleback Ferry, Fort Rannick, etc. Note: Magnimar and Sandpoint share noble families so moving the story backward in time might impact information about those families from other sources like the Magnimar sourcebook, and so on.
I don't see any major dependencies in RotRL on other golarion timelines. The setting is 'default' Varisia and nothing much changes in the land between the beginning and the end of the AP, at least as written.
I would also say it does sound suitable to the kind of 80s cult feel you are going for. You'll have some fun with the ogres in book 3, the ogrekin especially.
Latrecis is right about the 'primal' thing. If you want to Varisia to feel primal, you need to go back 400 years to before the Chelish settled! Varisia imho is supposed to feel like a frontier territory, with less than subtle parallels to the American frontier / wild west feel, plus added Roma gypsies. Then at the (puritan) settlers on the coast.
"Primal" was probably my poor non-English brains idea of wanting to say "In the beginning of it all" as opposed to something prehistoric =P
Thanks for the tip about the Magnimar Sourcebook. It does list a lot of historical points. I could perhaps move the -entire- story of Golarion back 19 years and adjust the events accordingly. I was thinking of doing something of the sort anyway, but it might create confusion if we are to start a different AP and, while not necessarily breaking continuity, we would have to use the "altered" timeline as a point of reference for all APs to come.
One plan involved having the central NPC Ameiko Kaijitsu be in her early 40ies during the 4710 Jade Regent =P
One of the reasons behind why I wanted to do this, is that a lot of the PCs from our Kingmaker campaign have backgrounds related to Varisia. I would love to acknowledge that background with references to their other PCs, adjusting plot points to involve them at a younger age, or possibly their parents. One of the players in RotR is playing the orc father of Davik, an NPC from Kingmaker for instance.