CoD will still sell more... I think.
I think BF will get closer than ever to takeing that #1 spot though.

The trailer for BF looks really impressive though.

That just means the youtube algorithms will give it all the more priority just like Justin Bieber videos.

overtake in sales or quality?

sales no... quality, it's always been superior and by a lot :)

Probably not, but you never know. It's easy to dismiss, because CoD has been king of the hill for so long, but nothing/nobody stays king forever. It's inevitable that one day another game will take it's place.

Battlefield 1 is gonna be huge. It Will most likely outsell Battlefield 4 and sell Close to Battlefield 3 which sold 17 million in one year ( and now is most likely over 20 million ).

That Said, it won't beat COD.

GTA and Minecraft are already bigger.

I'm aware, I thought we were talking specifically about shooters

Well maybe, but many Times i heard People who say COD is the bigger game in General, i don't even Know anymore who talk about shooters or in General. :p

BF will be lucky to sell half as much

Yes it will.
Battlefield 1 trailer: 19 mln views in 3 days.
Cod: IW trailer: 13 mln views in 7 days.
It seems like Battlefield got attention even from more hardcore audiences meanwhile Cod still lives in it's own bubble.