Slitherine

I really like the 'introductory' campaigns that each faction has. They seem to be 'not too hard', generally involving learning how to use the faction, taking a few provinces, building out the troops, etc, and getting completed in a reasonable amount of time.

The next step up, and there are 4 or 5 other campaigns from memory, is enormous. They are all 'take over the world' or near enough type deals.

I would prefer some campaigns that were at the same length, or a bit more, but well short of the enormous campaigns that are available now.

I agree, but I do think that for some nations the "conquer the world" campaign is unbeatable.
Nations that have no ships available will be unable to completely conquer the world.

I had this problem with Cor Vilaad after having conquered I guess about 25-30 percent of the world map, no ships so the sea nations like Ariselle or Azure Isles will be effectively unconquerable, considering the AI will not destroy another AI's capital.

All realms have naval potential. Annex a province with a Harbor in it. Most realms may then purchase naval units. Some realms (such as dwarves and orcs) may simply move their units to sea to convert them to armed transports.