I was just writing in another thread about weakening franchises like this one, their aging stars (Neeson, Stallone, Schwarzenegger), and foreign box office around 75-80%.
You should be thinking of Ford (possibly why you're here) also because this would be prequel with Bruce Willis bookends, set in New York 1979. Think Godfather III, The Warriors, Summer of Sam, Donnie Brasco. Like Die Hard 3 it sounds like an existing script with McClane shoehorned in.
I'm a big fan of the Die Hard series, but 5 was truly bad. In much the same way I didn't enjoy KOTCS I hope that if these franchises do make another entry that they go back to basics. Ditch the children and the passing of the torch themes, get back to the roots of what made those characters good and also make them vulnerable too.
I'm not 100% sold on the idea of a prequel DH6, but I'm open minded enough to give it a go, but I think Willis is so engrained as McClane that it could be difficult to pull off.
The Death Wish remake looks pretty fun. But there's no guarantee Bruce Willis will have a significant role in what's currently called Die Hard: Year One, loosely based on this comic book set in the 1970s.
But if you want to go back to the 1990s, a great archived film news site is Corona. Here's their Die Hard 4 page. There were many unrealized scripts. The most interesting to me was Tears of the Sun (a title later applied to another movie) supposedly set in a jungle. If Die Hard 6 is set in a jungle 40 years ago it would essentially be an Indy flick, right? Also:
There were two other potential Die Hard 4 scripts floating around at one point, both called Die Hardest, both written on spec by Ben Trebilcook, and both rejected. One would have been set in Tokyo, where McClane’s son worked for the Nakatomi Corporation, while the other was set in the Caribbean, and would have seen McClane and his daughter fighting shipwreck looters.