The coolest way they could do this is if they went the route of the first season of True Detective.
There are two John McClanes, the older one played by Bruce Willis (in the present day), and then a younger actor playing McClane in the 1970's. McClane in the 70's is a new rookie detective trying to solve his first huge case... but he never solves it. Intercut this with modern-day McClane still trying to solve it, thinking back on his earlier years when getting the initial leads and evidence, remembering suspects, etc.
If that's what they're thinking, I'm 100% for it and I think it's an amazing idea.
Look guys, it's Die Hard. It's Bruce Willis kicking ass. Just be happy we're still getting these movies at all, because it's only a fragile amount of time before we won't have Schwarzenegger, Stallone, Willis... Enjoy it! I for one welcome all this stuff with open arms. For me, these guys are the classic action stars, along with Mel Gibson and Van Damme. They're like the Bela Lugosi, Vincent Price, and Boris Karloff of the Action Genre. Let them ham it up in a cheesy bad movie or two. Or three. Or ten.