I'd love to see a prequel about how John became who he is today, of course Reeves is too old, but a prequel from when he was young and his journey to the first film would be cool.
I would be strongly considering that if I was controlling the franchise. Reeves is in good enough shape that his current middle-aged body would not pose a problem (like with DeNiro in 'The Irishman'). He has defined facial features. There is a big file of old reference footage to draw from. He seems like an ideal candidate to be de-aged for a whole movie.
A couple years ago they de-aged PeeWee Herman for a whole movie. They did it to Sam Jackson. It's not that crazy of an idea. Especially if they don't try to spend the whole movie too far back. They could write the first act when he's pretty young and then jump forward to finish the movie just a few years before the first JW movie.