I came across this article today on Tamriel Foundry (http://nullsignifier.blogspot.jp/2014/03/letters-from-tamriel-part-i-prophets.html), and I've been looking for something like this for a while. My personal opinion is that we see at least 4 million subscribers at one point (possibly in June/July), for all consoles combined. Whether or not that base holds will depend on how well ZOS follows through on content delivery every 4-6 weeks.
I'm curious, what do you all think?