Alright, some further suggestions:
Iroquios and Shoshone:
You can add the Totem Pole (Monument replacement) to one of the indian tribes.
Also the Tipi as an UI, there is even graphic for it in one of the scenarios (altough Tipi would be way more fitting to the Sioux, Shoshone used Wigwams AFAIK, but maybe Tipis are also good enough.)
Inca UB: Tambo, definitely. Some sort of post office and/or tavern type building along the Inca road system.
http://en.wikipedia.org/wiki/Tambo_(Incan_structure)
Venice: CBP use Doge's Palace, a unique Palace replacement as an UB for Venice. I kinda like that idea, as Venice is close to a One City Challenge after all.
Sweden: CBP use Skola for them (Public School replacement). Some other worthy UB ideas:
Trading Post/Longphort (Caravansary/Lighthouse), Soldattorp (Barracks/Military Academy)
IMO the best is the Trading Post (it can be represented as an UI too), as early Swedish vikings were closer to traders, than their Danish counterparts. They traded all along to Eastern European rivers, down to the Black Sea, made it into the Byzantine court as Varangian mercenaries, and later establised the Kievan Rus.
/The way I see it, and would represent them where the Scandinavians are not a single civ: Danish Vikings - raids, conquest, Swedish Vikings - trade, mercenaries, Norwegian Vikings - exploration, colonization. But of course everything goes for every group to some extent, this is just the most important aspect of them when going viking/
Songhai: I don't really like the Songhai in civ, the Mali would have been a much better choice.
It would have been much easier to represent them properly as a full civ in the game.
The Sofa and the Mandekalu Cavalry are both Malian units for example.
For the Mali I could give quite a few perfect UUs or UBs, for the Songhai I have no idea at all.
Anyway, if you have no problem adding further Malian uniques for the Songhai, I would choose from the following:
Royal Mint/Treasury (Mint replacement), Gold Panning Site (UI), Farari/Farima (Knight or Great General - they were an elite force of cavalry commanders), Kelekoun (GG), Kelebolo (Archer/Spearmen), or even the Skirmishers from Civ IV (could go as an Archer replacement)
EDIT: Ups, sry for the historic rants in the last two, got a little carried away
EDIT2: My post isn't too eye friendly, so here are the IMO best suggestions again:
Iroquios: Totem Pole or Sacred Pole (Monument)
Shoshone: Tipi (UI)
Inca: Tambo (would probably make it a Caravansary replacement, but would be great as an UI)
Venice: Doge's Palace (Palace)
Sweden: Trading Post (again, Caravansary or UI)
Songhai (which is Mali actually
):
Royal Mint (Mint)
EDIT3:
Oops, I just realized the Tipi is already an UI for the Shoshone
I'm not sure how was I able to miss that, sry