Word is that the Die Hard prequel/sequel hybrid that Live Free or Die Hard director Len Wiseman, The Conjuring screenwriters Chad and Carey Hayes, and producer Lorenzo di Bonaventura were developing at Fox a while back was sort of a dream project for franchise star Bruce Willis, who had apparently been talking about the idea for years. The approach was that the film would be cutting back and forth between present day, where Willis would once again be playing the role of John McClane, and December 31st, 1978, where McClane would be played by a younger actor. This would allow Willis to draw in another McClane-sized paycheck, while passing most of the action over to the younger actor. But it doesn't look like the project is going to happen, as Lorenzo di Bonaventura has revealed that Disney shelved it when they acquired Fox.
The producer told Polygon,
Yeah, no, it’s not happening. But what was really interesting was we actually came up with an idea to do it. It was a project that wasn’t Die Hard that then, eventually, shifted over to Die Hard. What was interesting about our idea was it allowed you the ability to meet the young John McClane and use Bruce. So it was really interesting in that way. So you sort of got to see both versions of him. A bit [like The Godfather Part 2]."
In 1978, the younger McClane would be working as a beat cop and dating his future wife / ex-wife Holly. Whatever happened at that time in McClane's life,
what we see in the ‘70s has ramifications on present day Bruce."
The part about the project starting off as something else and being changed into a Die Hard project is very much in line with the rest of the franchise, as A Good Day to Die Hard was the only sequel that originated as a Die Hard script. Which is ironic, since it's the least "Die Hard" and most generic of the whole bunch.
After A Good Day to Die Hard and after watching Willis sleepwalk through multiple films, I didn't have high hopes for this prequel/sequel (which was being called Die Hard: Year One and McClane at different points), so I feel like Disney might have helped Die Hard fans dodge a bullet by scrapping it.