If I was given the choice, the one region I'd want to have a go at would be the Hanseatic League, though it probably wouldn't take up enough space to fill a handbook by itself. (Perhaps as some sort of PDF-exclusive digest?)
From the first movement of people and material in the Lyran Commonwealth which allowed the first colonisation of Hansa space to happen, to the birth of the Hansa as a political organisation (and whether or not it carried any of the relics or regalia from
the medieval League, or of
the modern-day Parliament), to the gradual expansion of the League's borders (and the emergence of its long-range trading routes across the Deep Periphery), right up to the post-Reaving headaches caused by the political turmoil in Nueva Castile and the sharp increase in interest towards the region by the remaining Homeworld Clans.
Ideally, it could include planetary maps of key worlds like Bremen and Antwerp (both of which have planetary data recently featured in
Objectives: Periphery) and the three "open" planets (Bergen, Bruges, and Lübeck), along with star maps showing the gradual expansion of the League's sphere of influence (akin to the early maps of the Marian Hegemony in
Handbook: Major Periphery States). A more detailed look at the six Regional Defense Forces would be in order, as would the somewhat ironically-named Convoy Defense Force.
There'd be a look at the major players on the Council of Merchants, the risks and rewards of citizenship, and the depths of serfdom in which those who fall too far down on their luck end up falling. (And how this is reflected politically. Is there a strong abolitionist movement on any worlds in particular, or across the League as a whole; or does the Council's control over JumpShip traffic choke off attempts at societal reform, unless any of its members happen to champion the reformers' cause?)
And with the dramatic changes facing the Hansa by the mid-3090s, can the logistical and military status of the League make any sort of adjustments from the state both were left in as of
Field Report: Periphery and
Objectives: Periphery; or is the League fated to be written about in epitaph in the Star Adder Remembrances of the Dark Age era?
It is a shame that the Hansa seems to have its card marked just as things were starting to heat up in its near neighbourhood; while I could understand the in-universe reasons why
ISP3 was mostly silent regarding the League, I still found its omission to be a missed opportunity, of a kind that comes along all too rarely for the Deep Periphery states. So, even if the powers that be plan to turn Hansa space into a new Star Adder dominion by the time of the Dark Age, I would like there to be at least one place where the League could be fleshed out in more detail.