Jokes aside I would really love to see agriculture be expanded upon like food supplies in general. Something to split development into population and economical output maybe, a big population doesn't equal high economic output all the time but they need food, it would be nice to see it lower in war especially if sacked and farm can produce food for faster growth of population. Though this may be needed in a plague update, I would love to see it.
Honestly, I'm actually really pleasantly surprised with the small but notable number of people here who've said they'd genuinely really like more agricultural content.

It's always good to be able to point to this type of thing when discussing future plans, and though I'm definitely not in the Add Pops to CK3 camp that's occasionally chatted about on the forums, I am pretty interested in the type of thing you're talking about here - mechanical consequences to agrarian societies for lengthy wars, a more flexible representation of prosperity than the development system currently gives us, and economics that are a
little more detailed without going full trade-sim.
I've also always felt that, especially for minor land-owners where it would be a more pressing concern, it'd be neat to have more content related to bad or good harvests sweeping the realm, boons from the advance of agricultural techniques, changing global temperatures, and so on. ^^' Though I don't think we'd ever do a DD on agrarian science
research, I wouldn't be complaining if I was in a position where I had to write a DD on a handful of new agricultural mechanics and events.
At first I thought, the reference to the thirteenth century was linked to agricultural advances. But if we depart from the supposition that this dev diary is not actually about agriculture, but that it is a code or something, then I wonder what the "thirteenth century" reference means.

Thirteenth century is best century, that's all. Edward I, Ban Kulin, both the Latin Empire
and Michael VIII, the list goes on. It's a fascinating era in history and I really wish we had a bookmark in it. Or two, or three.
... sadly I think 1200s bookmarks are
far down the bookmark prio list, after our fan favourites in 936 & 1081. I guess there's 1187, which is close but suffers from a chronic case of Richard the Lionheart being alive.
Is this just an artifact from the CK3 devs borrowing CK2 code? Or does the comment suggest that at some point during CK3's development a 1220 start date was on the table - a bookmark that was ultimately cut but will make its return with the next expansion?
Though CK3 generally doesn't include reused script from CK2 (except for some of the loc of the health events, I believe), the major exception to that is our history files. They're not 1:1 the same, as CK2 continued development for a while after CK3 started up (which is partially why late history changes introduced into CK2 sometimes aren't present in CK3, though there's also sometimes a deliberate design choice there too) and CK3's done a decent amount of redevelopment and revamping since, but you'll find a lot of crossover from that shared heritage.
The biggest part of that is CK3 retaining the post-1066 history, even though we don't have a post-1066 bookmark. As far as I'm aware, sadly, there was never a 1200s bookmark on the table, but there were (and still are from some members of the team) hopes to have some specific later bookmarks at some point in the future.