Source: The Wrestling Observer Newsletter
For those wondering, Sting is not expected to be a surprise entrant in the 2015 WWE Royal Rumble match. As things stand now, the plan is still for Sting to work a match against Triple H at WrestleMania 31. If those plans end up changing, we should hear about it in the near future.
As far as Triple H and The Authority are concerned, apparently the plan was set for some time to bring them back, and the decision to bring them back sooner than originally expected was said to not be a reactionary move to recent declining ratings.