what if Micheal Douglas played Martin Riggs

He would've gotten throat cancer by eating Mrs. Riggs snapper..

What if you were literate and knew how to make a topic, complete with punctuation and, y’know, actual thought?

What if your father used rubbers?

And you are Master of what?

There probably would not have been sequels.

He'd have stuck his 'Lethal Weapon' in more (and better) trim than, Patty Kensit and Rene Russo

He did, in Black Rain

According to Hollywood agent Michael Ovitz, Lethal Weapon was once pitched as the third buddy movie for Robert Redford and Paul Newman to star in.

He also lacks the humour that Mel Gibson brings. The film would have been very different.

It wouldn't have been the same because one of the themes of the Riggs/Murtaugh partnership dynamic is that Murtaugh is older and wiser than Riggs.

Having Riggs be closer to Murtaugh's age or older would have made it a different movie.