The English future temporal reference system has long been recognized as a variable system undergoing change. The main variants in contemporary English (will and be going to) have both been argued to have gone through (and to potentially still be undergoing) grammaticalization. At the same time, be going to has been gradually increasing in frequency relative to will over the last 500 years. However, investigation of the ongoing development of this system has been sparse. This article makes use of a large contemporary sociolinguistic corpus of a mainstream variety of North American English and the apparent-time construct. Several factors that have been implicated in the development of this system (Sentence Type, Clause Type, Proximity, Verb Type, and the Animacy and Grammatical Person of the Subject) are considered and a multiplex series of changes are uncovered. Underlying an overall, albeit slow, change in frequency towards be going to, we find evidence for specialization of one or the other variant in different linguistic contexts, neutralization of a constraint consistent with ongoing loss of variant nuances through semantic bleaching, and the persistence of constraints consistent with morphological doublet competition.