The Monsters Inc coaster is a rumor that's been around a long time. IIRC, the rumor started about ten years ago when there were reported whispers about a new Monsters Inc inverted coaster at DHS based on the door scene and that would be family friendly. The idea was linked to the success of Toy Story Midway Mania and the growth of Pixar in the parks. The coaster obviously never happened - and the fact that they just finished Toy Story Land and have the Slinky Dog Coaster instead doesn't suggest it'll happen anytime soon (if ever).
Over time, the rumor has morphed from a new coaster into an overlay of Rock n Roller Coaster. While it's an interesting idea and the proximity to Toy Story Land might make that plausible (plus ongoing rumors about the Aerosmith licensing), I have a hard time seeing it ever happen. Setting aside having two Pixar-themed coasters in one park, another reason is that Disney tends not to use young kid friendly properties to theme its more aggressive rides (as above - the original Monsters Inc coaster was supposed to be more SDD/7DMT, and less RnRC/EE). Most of the roller coasters across the parks have more "adult" theming (Indiana Jones and Tron, for example) and most aren't based on any particular IP at all. The coasters that are more kid themed (7DMT and SDD) are family coasters on the low end of the thrill spectrum. The only exception I can think of is the Incredicoaster - but on that one you know exactly what you're getting into since the ride is all out there in the open to see (unlike RnRC).
The point is - it's hard to see Disney theming one of its more aggressive coasters (that has one of the most restrictive height requirements) along the lines of a property that has high appeal among young guests that either couldn't ride it or have families that might get a much different experience out of it than what they may have expected from an IP popular among a young demographic (along those lines - imagine converting Tower of Tower into Tigger's Tiger Bounce).