There's a fair bit of ambiguity about whether or not the next coaster will open in 2015 or 2016 in the plans..
This is a screen cap from the Snoozebox applications.
The generally referred to (in the MTDP) "2015" coaster can go up to 50m, and would either go on Island A (next to Swarm / behing Stealth) or Island E (where the Snoozeboxes are). It does however say that the "2015" coaster on Area E may not be built until 2016.
The thing is, the Snoozeboxes are going to be there until February 2016. If a coaster was to be built for the 2016 season on Island E, it would of course be constructed during 2015 - would Snoozebox guests want to sleep on the same plot of land as a construction site? No. So, really, a coaster wouldn't open there until 2017 at the earliest.
The fact that it says the Snoozeboxes won't affect the coaster development would surely mean then that they'd want to use the island next to Swarm for the next coaster (which also makes more sense considering how obvious it is to everyone from Swarm)? There's nothing to suggest that the coaster on Island A is to be delayed per se, but it doesn't seem exactly convincing that the next coaster will be in 2015. I do remember reading somewhere else on the Snoozebox application that the park do want to focus of their accomodation strategy though, which could be a reason for delaying the coaster for a year.
The application for Swarm's billboard stated that there had been a poor response to marketing (despite actual ride reaction being positive), and so the "thrill factor" of the ride had to be increased, as so to bring more people to the park. Whilst the park aren't doing as well as they'd want to be doing, they're more pinpointing that to a poor response to marketing, as opposed to actual failures of the park itself.
As for what the next coaster will be, there's absolutely no indications whatsoever what it could be. The MTDP only shows mock layouts to give an indication of size to the council and there's no rumours really about deals with manufacturers or anything like that.