Well... Argus came out of nowhere, so they are not above pulling a fast one. Sargeras wasn't the final boss.
I want Sylvanas to be the final boss. I want N'zoth and the old gods to be the theme of the next expansion.
But, lets get real. Sylvanas is gonna get shoved aside in 8.2.5 and N'zoth will get flattened in 8.3 and we will get a happily ever in Silithus at the end... until whatever 9.0 is. Probably death related and we will get no tinker again, cause blizz loves to disappoint.