Joe Montana Football 16 is being built on Unreal Engine 4, assuming it really exists.
Joe Montana Football 16 officially does not exist. Whichever publisher and developer is working on it, they're not talking. The only way we know about it in the first place is because Montana himself keeps teasing it.
The latest update from the former Kansas City quarterback is that the project is being built on Unreal Engine 4:
— Joseph Montana (@JoeMontana) March 17, 2015
To recap the story so far: Montana has been teasing the project on Twitter since July 2014, and so has Damon Grow, an experienced developer working for a "super secret sports company". Operation Sports has a good rundown, with links, of the info the pair have released, including denials of EA's involvement.
Coincidentally, a fresh crop of Joe Madden Football 16 rumours cropped up lately. These are all very much in the "some guy on the Internet says" pile, but according to a recent Operation Sports forum post by an anonymous user, the game is coming to PC and Xbox One with cross-platform technologies. It will be revealed three to four weeks ahead of its release in mid to late July, and leaks to date have been part of a stealth PR campaign.
The poster went on to say it's an officially licensed NFL game and will have that in the title, in contrast to the title supplied by Montana himself. A previously planned non-licensed football game built with community involvement was cancelled. There will be a "really deep" mobile app for franchise mode management, and Joe Montana's mobile app will not be coming to market. 2K is "involved" somehow, supplying technology for development. The story mode is said to be like NBA 2K's My Player mode.
Take all that with enormous piles of salt, obviously.
It'd be very interesting to see 2K step up to challenge EA's NFL dominance, given their NBA rivalry. We've asked 2K for comment but aren't massively hopeful of an exciting response.