well as far as we know, the lion turtle aang saw was the last one
and this story is going to have many antagonist, because like i said, this will be a very long term story. i know Ash even hopes taht this goes long enough that we may even end up doing the /next/ avatar after mine as well, heh.
so if i understand Krad He's an airbender, but has not been with the airnomads either. If our adventure is beginning in the southern water tribe, would Krad have wandered that far south? Also, some how the Dai Li always end up secondary antagonist..lol
for now I would assume spirits still run free. Korra never closed the portals off again, and i don't think there would be a reason for her to in the upcoming book. I am though hoping that Korra finds a way to reconnect with the past avatars. we know its not impossible because Aang ruined his connection to past lives too and some how it was restored [popular theory is it may have had something to do with Yang Chen's festival] Lion turtle's Arent something to be counted out though, there could be a couple others that still exist, much like the one Aang saw.