Definitely not a patch 6.6. .0 .1 .2 .3 .4 .5 .6 are all "major releases, with .0 being the expansion itself. These are MSQ and new gear/savage/tomes tier (even releases) and 24-man (odd releases). If 6.6 comes out, we'd see more MSQ and another gear tier to keep with the release nomenclature they've followed since ARR. That's not to say we won't see 6.55, 6.56, 6.57... 6.59 instead.

SE is very formulaic with their content release. They haven't strayed before and I doubt they will now. If the expansion is released late (by our perception) then it will be released late. That's all their is to it.