Uh, you can grind my post down to the finest powder and sift it with the finest sieve and you won't find a single mention of "evil", although Unalok was aware that Vata utilized the Sith Smorgasbord of hate, fear, anger, etc. so it seems like the 10,000 years of Darkness would have been slightly less than fun for all involved.

Also, I can kinda see your point vis a vis the spirits, but I don't think we've ever gotten any indication the Avatar was responsible for ensuring the division of people's. like, it seems much more de jure rather than de facto separation, and Wan seemed to die trying to make people stop fighting after they had started to mingle, not battling them back to their respective corners.

Also also, after watching Wan's episode again, I think an entire season of him is unnecessary. The beauty of that story is its simplicity and legend building, no need to muddy that up with needless detail. However, can you imagine what it must have been like for the second Avatar, suddenly being thrust into this war torn world saving role with none of the infrastructure that Aang or Korra had for training and such, with only one past life to make sense of it all? That'd be something to watch.

Basically, bring on Avatar: The First Airbender.

I haven't watched the second season so I might be out of the loop, but I think the Avatar has been portrayed as responsible for maintaining if not the originating the division of the four elements into kingdoms. It was explicitly Avatar Roku's duty to prevent Sozin from colonizing Earth Kingdom territory.

Yeah, I imagine it may have actually been the second Avatar that established or at least laid the seeds for what would eventually become the training infrastructure that Aang and Korra both went through. Would be an interesting story to see.

Mostly this
I dont know if you've read the comics, but Aang's first order of business after the war was to kick out all of the fire nation from the Earth Kingdom. Keep in mind the earliest colonies are so old none of the original colonists are still alive and every single one of them was born there. And to further add to that JUST the fire nation decedents. The Earth Kingdom decedents living there are expected to stay.

Obviously in the end Aang didn't do this but its clear he was expected to.

I am betting there were several generations of the Avatar struggling and fighting to create some form of stability. Keep in mind even if everything was all peaceful and people were all for helping him the martial arts that became the bending styles weren't created yet. One reason Wan was such a skilled fire bender is from learning and honing the style from the dragons (seeing him do the dancing dragon was such great fan service). My guess is it would be much like Exalted, with the new Avatar having to rely mostly on past life memories to learn how to bend

I do agree it would be fun to watch if just to see how the world was early on.

Still my vote is for the Avatar after Korra. I want my Avatar: modern

I guess, but I thought that was more along the lines of "Hey, guy, don't go acting all imperial and starting shit," rather than "Stay within your boundaries or else." I mean, if it had been more of a Louisiana Purchase/mutual thing with no way to incite war I don't see how that would be upsetting the balance or whatnot.
Nope, haven't read the comics. That all sounds p stupid.

Also, what do you mean by "expected" to? From what I can tell, especially after Wan's story, being the Avatar doesn't really come with set of conditions. That is to say, Wan just sort of declared himself the keeper of the balance, but have we really ever gotten any indication that the Avatar state can be revoked if "the balance" isn't upheld rigorously enough? What, besides some past life scolding, is really stopping an Avatar from becoming a despot and ruling by his whim with his awesome power?

Roku's exact words "The Four Nations are meant to be exactly that, Four Nations", and this wasn't his reaction to talks of war. This was his reaction to Sozen simply talking about 'spreading fire nation culture and prosperity'. Roku wasn't freaking out at talks of war, he was freaking out over talks of cultural expansion.
obviously not, cause he totally didn't do that. Expected to as in from a societal/cultural view point. People expect him to do that just as they expect him to calm spirits or stop pyromancer Hitler. As far as they are concerned this is just something the avatar does

Not saying Wan intended this, but I'm sure there was some Avatar down the line that essentially said "you four different cultures pick a corner and stay there". After all I doubt its a coincidence that for the most part each nation is completely comprised of one type of bending.

Well sorta. Didn't they run into those swamp people water-benders while traveling in the Earth Kingdom? And then there were the not-really-air-nomads that were traveling through all sorts of places.

They weren't air benders and the swamp people were isolated and unknown. Still not mixing with the Earth Kingdom. Remember they even stopped attacking simply because Katara was a water bender "that makes us kin". They associate themselves more with fellow water benders than they do with others living in the Earth Kingdom.

My biggest issue? If giant glowing Korra was what they did before they had bending... Why the heck would they change over to bending and never do that again?

Plus, if she doesn't solve every problem from now on like that, why?

Either A) Because the full moon or whatever bullshit allowed her eleventh hour super power to come into effect is gone now or B) For the same reason the Power Rangers don't call in the megazord the split instant the monster shows up.