I chose "too hard to predict," but I suspect that the country will end up being divided into regions along religious/sectarian lines. In the long run, it may even split into three separate nations: Sunni, Shiite, and Kurd.
Hard to say, because Iraq isn't a war; it's just one battle in a much bigger war. We can walk away from Iraq (I guess) but we can't walk away from the whole Middle East. We'll probably stay in Iraq for decades while we struggle with Iran, Syria, et al. Iran will probably get nukes and then push on with its plan to conquer all the oil producing states of the Middle East. We'll resist them and there'll be various shenanigans with the Saudis and others who don't want to get taken over by Iran.
Actually, walking away from the Middle East is exactly what the US should do. We should have no dealings with them at all except buying their oil at a fair price -- and, perhaps, being an
even-handed broker for peace in the Israeli-Arab conflict (rather than always taking Israel's side). Hopefully someday we'll no longer depend on that oil.
I doubt that there's any evidence whatsoever that Iran is planning to take over the Middle East. As anxious as Bush et al. are to attack Iran, and as much as they're looking for an excuse to do so, I haven't even heard them make that claim.
Of course we should keep an eye on Iran and on all countries, but we shouldn't meddle in their squabbles and make more people hate us and want to attack us in the process. If some Middle Eastern nation launches a military attack on us, then of course we should retaliate; but apart from that, we should leave that hornets' nest alone.
Iran probably does want nukes, and I honestly can't blame them. Israel has nukes already, and the only way a small nation can secure itself against a US invasion is by having nukes. Besides, what right does the US government have to say which nations are allowed to have nukes and which aren't?
Short answer: We'll maintain a presence over there for a long time.
That's what the Israel-firsters want, but I hope they don't get it. The reason they pushed for an invasion of Iraq was to redraw the Middle East and "re-educate" Muslims in a manner favorable to Israel, but at US expense in blood and tax dollars. Here's one Israeli think tank on the issue (from 2002):
To democratize Islam it will be necessary for the United States to conquer Iraq and other Islamic regimes and maintain an occupation force for two or three decades, as was done in post-war Japan and Germany.
A generation of Muslim children will have to be re-educated. Anti-Jewish and anti-Christian verses in the Qur'an should be neutralized by contrary verses and commentaries. The principle of Jihad must be eliminated from the four schools of Islamic law. Islamic regimes must abide by the Seven Noahide Laws of Universal Morality.
Source:
http://www.acpr.org.il/publications/policy-papers/pp141-xs.html
The PNAC and other neocon outfits revealed similar plans in their pre-war writings. All of it is a matter of public record and is available to anyone who wants to search for it.
The neocons truly pose a menace not only to world peace but to the security of America, in spite of their pseudo-patriotic jingoism. We should be focusing on sealing our borders and more tightly controlling immigration (especially from potentially hostile countries) rather than continually stirring up the hornets' nest and leaving ourselves open to being stung.