You said in your 4th thought that;
" It's suggested throughout Infinite that Columbia's bigwigs have derived much of their technology from Andrew Ryan's submarine dystopia. ".
Um.. No, it would be the other way around. Rapture was mostly finished and being inhabited on November 5th 1946 to January 1at 1959 when the civil war broke out. Columbia was created in made 1893, and still in the sky, this is 9 years before Andrew Ryan was born and many more before he left Russia.
Does that mean that in 1940, there will be a city in the clouds with barely anyone there and most of the skylines broken? I do wonder what will happen to those people that couldn't escape Columbia.
Also, it's pretty clear the Booker joined the Vox Popili because Elizabeth got taken away or else why isn't there a Voxaphone of her talking about losing Booker? That would have made more sense, Booker wanted to stay out of the whole civil war, but he wants no harm to come to Elizabeth. He went threw hell to get her and he died trying to get her. That just how one Alternate universe ended. Maybe there is a universe where she still has her Pinky or the Coin Flip changes. Honestly, I don't care about Comstock's rise to power. He's a racist bast'rd and it made the game real easy to clever through those narrow minded people.
There are a few problems with this WIshlist of DLC:
1. A conclusion where there is no Comstock, only Booker DeWitt would actually be awesome to see in one of the Sea of Doors.. If there is no Comstock, there would never have been a Columbia. It would most likely be a different floating city, if at all, under a different name, different tyrannical leader, but lacking the GIrl whom can make tears, and the twins would most likely have never been stuck ghosting between dimensions, but still possible. They seem happy together.. Except for Robert. It might, arguably, lead to an actual 'True Ending' where there was no Comstock or Columbia leveling out the Boxer Rebellion..
2. As interesting as Songbird's origins would be, you get to see the plan of him. It's a human in a suit, but probably as much of man as the Handyman are. It's not like in Bioshock: DLC of Sigma, where out get to learn about Charles Porter was made into a Big Daddy, that was sad as hell. I do feel bad that unlike in Irrational's Bioshock, where they tell you about the story of the Enemy Class characters you're fighting, Infinite did very little on that as to tell a bigger story, but it still felt off. Less spending on marketing budget, focus on the game. You could put out a ' Hey this is coming out on this day' right on their site, Xbox LIve and PS Plus, and people be foaming at the mouth to get it. I hate how I got the soundtrack, but I didn't get the 'Tainted Love', 'Girls Just Want To Have Fun', and 'Making Whoopie' songs that was Easter Egged around on record players. I laughed so hard.^_^
I have a few more points, but it is late and I let my Hardcore Bioshock nerd flag fly high enough for today... Mm.. I need to get a job in story design. >.>