MTW Pocket Mod: Units, Buildings and Unit Stats

[caravel]

...also make the defense upgrades (ie curtain wall and ballista towers for the keep) mandatory before you can upgrade to the next castle level, as well as making these defenses somewhat more expensive. This will prevent players from upgrading to citadels and fortresses too quickly.

Not sure about this. This won't stop the AI from doing what it's doing. It will still advance through those tech levels in order to get to it's objective, in this case +1 pikemen. I'm confident that if I had made all upgrades part of the tech level, the French would still have raised that province to Citadel abd still teched up to pikmen.

For the royal courts: maybe it's possible to insert a new agent type that acts like an improved emissary, that's trained at the royal court (just like Cardinals and Orthodox bisshops are basicly just improved bisshops and priests)
Alternatively it could be used like a prerequisite for the Chancellary and some of the other higher-up government buildings.

Improved emissaries won't work. The only difference between, i.e, bishops and cardinals is the faith propogation AFAIK. Emissaries don't propogate faith so their abilities are unchangeable. The only way to improve them is to have building upgrades that increaese their valour (as is the case with assassins and spies). So it may be possible to use the court buildings to increase the valour of emissaries. Though personally I would use buildings such as the chancellory, admiralty and university etc for this purpose.

Why is "lancer" an unrealistic name? It's kinda bland, but not moreso then "plated knights" or anything else I can think of at present.
If they're going to be added to all factions, I do suggest that the valour bonus in an Iberian province is indeed implemented.

All knights are pretty much Lancers, it seems strange to have a tech tree of: Feudal Knights -> Chivalric Knights -> Lancers. The Lancers in MTW are also fantasy, and shouldn't be restricted to the Spanish. Lancers in the true sense, is how the later lance wielding cavalry of the 18th century are often described.

the idea about additional requirements sounds good. However the question is if the AI will build them on its own, since fortification upgrades usually do not enjoy a high priority with the AI. And I don't quite get the point why players wouldn't want to upgrade to a fortress in the Sahara if fortifying a province actually becomes cheaper.

The AI will build them if the build prod file is modded to encourage them to do so, and should do anyway if they have to build them to build something else.

An upgraded emissary also sounds good, it would be interesting to see, though, as to how this could be reflected in-game. The difference between bishops and cardinals is mainly in conversion speed (and they don't profit from valor levels except against assassins), the distinction between mere inquisitors and grand ones in burning chance and range of targets. Since of all the emissary's duties only one seems to be tied to valor afaik (bribery missions, at least that's my impression), it would maybe suffice to provide new emissaries with a valor bonus, similar to the one given to spies and assassins. Given the emissary's role in the game, I'm having difficulties in imagining a broadening of his competences in an 'upgraded' version.