shipwright or shipwrong: mod 'em if you can

[drone]

Does it mean that they are unmoddable? So modding amounts to introducing new units, new factions and new provinces, does it not?

Depends on what you mean by "unmoddable". You can't alter the skills/limitations of the "Ship" agent type, but you can create new ships or modify the stats and prereqs of existing ones.

[Gilrandir]

You can't alter the skills/limitations of the "Ship" agent type, but you can create new ships

So, you can create new agents? Muslem princesses? Mendicant monks? Leprous beggars? Individual Holy Grail seekers? Peripatetic troubadours? Any other legendary characters who purport to have filled medieval roads? If so the campaign map can really become way too overcrowded.

[caravel]

There are set agent types with set abilities. You can create an agent called a "leprous beggar" and base it on the assassin, but it will still function as an assassin...

Muslim princesses are kind of possible but you have to set a building up to train them as the game won't spawn them automatically. This is what I did with agents in my mod ( as far as I can remember ):

1) Harem women (muslim princesses)

2) Experimented with spies/assassins who spread heresy (you can change the faith propagation for all units and buildings)... (this was interesting as it forced the player to use bishops to counter it and made the abuse of spies more difficult - it was only a small percentage and I think churches/mosques were enough to reverse it).

3) Invisible Papal Inquisitors (these were interesting because they removed the Inquisitor agents from the player and turned them papacy only - an unplayable AI only faction by default - thus making them more of an uncontrollable natural disaster - but more importantly because they're invisible counterspying keeps them under control - I can't remember how well it worked...).

So you could have your "leprous beggar" spread heresy and add a support cost (alms), but he would still be an assassin or whatever agent you based him upon...

//edit: The most versatile agent type is the "priest" types. With these you can create some rebel only pagan priests, rabbis, etc, to add flavour and a bit of randomness if nothing else.