First in the series AAA IP, hasn't been in PS+ (that I know):
Assassin's Creed
Batman: AA
Bioshock
Castlevania: Lords of Shadow
CoD4
CoD:BO
Crysis
Demon's Souls/Dark Souls
Dead Island
Dead Space (not likely)
Dragon Age Origins
God of War HD (i have the weird feeling that it is this one)
InFAMOUS (but it was part of the Welcome Back in 2011... same with LBP)
Mass Effect
Uncharted: Drake's Fortune (it is on the store)
Maybe I left others behind, but those may be
1) AssCreed III was on PS+, for EU. It'd be weird if NA got AssCreed 1. So I doubt it.
2) Batman: Arkham
City has been on PS+, so I doubt we'd get Asylum.
3) Ditto with Bioshock Infinite
4) Mayyyybe. That was one of my guesses.
5) CoD4 isn't the "first of a series" xD Unless you mean MW 1...
6) Ditto
7) We got Crysis 3 last month.
8) Demon's Souls was on PS+ in April 2013.
9) Maybe.
10) Dead Space 3 was a July offering.
11) Maybe
12) Maybe
13) Doubtful for the reason you stated
14) Maybe
15) Doubtful, we already got Uncharted 3
With that in mind, I'd say it's more likely to be one of those:
Castlevania: Lords of Shadow, Dead Island, Dragon Age: Origins, God of War HD, Mass Effect
Good guess on God of War, but I hope that's not it. I'd like Dead Island best out of those, personally. I already have the others on other platforms and don't care about them anyway.