PoP doesn't need Diplomacy, imo.
No mod needs Diplomacy.
Diplomacy is like a venom: penetrates every script, every trigger, works as one system rather than many different improvements. Some parts of it work fine, some parts don't, but it's hard to separate them and cut the latter ones (or not add them, if we are still at this stage).
Diplomacy is often overcomplicated, going too deep into economics or flooding triggers with large try_for_range cycles where they are not really needed, eventually creating lags on the global map.
Many changes are unnecessary or insignificant, like the cases where, based on kingdom's settings (like serfdom, troop quality etc.) variables are altered by +/-3% max. (3 f****** percent!). Yet they exist in the code, make it larger, messier, harder to navigate (and to process by CPUs).
Also Diplomacy was created for Native, and has a lot of code that deals specifically with Native troops, lords, kings and factions. All these parts should be identified and removed before adding Diplomacy to any other mod.
As a person working on Perisno that already has OSP code from Diplomacy, Silverstag, PBOD & Battle Formations & Combat AI and whatnots (with Diplomacy being the best one of all these large OSPs), a mod where almost every script is polluted with new unclear code (at least it is marked, so I know which part to cut if something doesn't work as it should), I envy people who work on mods that don't have all this mess in their files.
There are better, simpler, less "venomous" alternatives to almost every feature Diplomacy or any other large OSP (larger than, let's say, 1 specific presentation that was made for 1 specific purpose) has to offer.