There are missions to conquer pretty much every nation in Europe, he even said so in the previous post.
*EU3folder*\mod\MPM vX.X\missions
e.g. spanish_missions include missions to conquer every Iberian nation (including Navarra) and they're given to every Iberian culture nation excluding Portugal (with Galicia being an exception, since Portugal historically had some claim to this region, and even owned parts of it for some time if memory serves, they're also able to get the mission).
example:
Code:
annex_navarra = {
type = country
allow = {
OR = {
primary_culture = castillian
primary_culture = catalan
primary_culture = galician
primary_culture = andalucian
}
is_subject = no
is_lesser_in_union = no
exists = NAV
NAV = {
is_subject = no
is_lesser_in_union = no
neighbour = this
not = { num_of_cities = this }
}
}
...
So basically let's say Castille ends up being a fractured mess, lots of nations released. Leon may get missions to conquer and obtain cores on it's neighbour states and eventually with some luck being able to form a new strong state in Iberia.
edit: I'm actually not sure if Leon has primary_culture = castillian now that I think about it but you get my point (hopefully). There are missions like this in every region.