Hmmm, good points have be made by all!
I agree that there would've been a babyboom in Gondor after the War of the Ring. After all, humans tend to behave like bunnies after a major war. This would improve their population numbers though I doubt these babybooms could've provided the necessary numbers to recolonize Arnor, Dorwinion, Harondor and Ithilien all at once. Ithilien and Harondor are indeed easily reincorporated into the Kingdom, being close to Gondor's powerbase and having natural borders which are easy to defend. The Umbar situation still is a problem though, since Umbar and it's surrounding lands are already occupied by people hostile to Gondor. Military occupation is the only solution there, I fear.
Dorwinion is currently abandoned, but it lies far from Gondor's powerbase, beyond the barren lands of Dagorlad and it's eastern border has no defensive features to speak of. Once the Easterlings have recovered it will be beset by the same problems that plagued Gondor in the days of the Kings. Constant invasions from the east, which will in the end result in Dorwinion being abandoned again. One solution to this may be to actually allow peoples from the east to settle there and becoming something like the
foederati of the Roman Empire. Allied tribes who are allowed to settle in Roman lands in exchange for military service.
I also agree with
Anguirel about the splitting of the Northern and Southern Kingdoms. It is clear that the Reunited Kingdom is a cumbersome beast and eventually it will have to split. It is simply too large to be ruled effectively by a single man. I reckon the Reunited Kingdom will be ruled by no more than 5 kings before it is split in two again, just like the Roman Empire was divided in an Eastern and a Western Empire.
Lalaith 's thesis that Aragorn's Kingdom was more a loose federation of states nobody was forced to be a part of is interesting, but doubtful. Lets take the Dunlendings for example: What would Aragorn's response be if a delegation of Dunlendings would come to the Big Tree (as Minas Tirith would be called affectionately by Gondor's citizens in the 4th Age
) and say:
"You know, this whole Reunited Kingdom idea, it's cute, but we don't want to be a a part of it any longer. From now on Dunland is an independent state"
There is no doubt in my mind that Aragorn would mobilize his armies and call in his proxy Eomer as well and crush the Dunlendings for their impudence. Why? Because Dunland is the main link between Arnor and Gondor. Geopolitically an independent Dunland is simply out of the question, because of it's strategic position.
The same goes for the people of Umbar. I believe it is highly doubtful that Aragorn would give up his claim to the city (and it's immense strategic value) simply because the Umbarians (is that the right term?
) want to stay independent, despite Aragorn's claims that it used to be part of Gondor.