quoting Rhetorica:

Then let's do that—we can have a western region, as Hurq outlined, and an eastern region, in the island chain(s) on the other side of the mainland. I dub these pseudo-Peleponnesos and pseudo-Nippon, respectively.

I think that all too often collabs become pastiches as opposed to cohesive works. I believe, furthermore, that this tendency hurts the very idea of a collab, and turns it into more of a sidebyside aparthaid conworlding thingamajig: I fear this will happen to CCC over at the zeeb.
It is in this spirit that I can think of a good compromise: how about we start with two much smaller loci... (though one would be fine) but also decide that there are as many others as we want: I mean, Earth by this time period had a shitload of civilizatory centers: China, the Middle East, Greece and Northern Africa, at the very least. 

I propose that PP be restricted to this bit.

I mean, that bit is rather the same size as the Middle East, is midway from the poles to the equator <as Europe and in general primary civilizatory centers are> and also should have enough climate diversity.  <I really ought to get around to figuring out where has which climate... unless someone else wants to>.

Proposed tinier PP vs. afroeurasia

I think its quite large enough. That and only one of the big neojapans <of which there is about two, each one twice as long as Japan> ought to give us enough room for quite a bit of time.