Nibali and Valverde have won GT's, or are you only talking relatively recent times (and therefore more realistic future winners)?The problem for Almeida is that even if he takes that next step, it's still an awful route for him.
Barring him? We only have four former GT winners here. Two of those (Dumoulin and TGH) are almost certainly not winning this and Yates is flaky so it's hard to look past Carapaz, but I definitely agree that the circumstances for someone surprising stepping it up and winning the whole thing. Would love for that person to be Bilbao or Landa but I can't see it.
I find it almost impossible to see Nibali in contention, but Valverde you never know (stage race form still appears pretty good).