There are some good elements here, but overall I'm really not a fan of it, sorry. Though, with that said, it's miles better than anything we've had post-P2.
I don't like the 'Dutch when to live a lonely life' trope which a lot of fanfic/theories seem to bandy about. I don' think it suits the character.
Also, wouldn't any predator seeking revenge technically want to go after Harrigan? I mean, he *actually* killed one in armed combat in front of its peers/elder(s). Dutch merely forced one into self-destruct mode.
The way to save the franchise after The Predator is definitely to get Arnie back. He obviously didn't want the cameo offered to him by Shane Black and, I think we all feel he could've brought more to that film (or any other). But he is how you save it.
The one thing the OP does touch on is the 'boxing film' trope. That is definitely how he needs to be used in a future film. He's old, he's past it, he can't win by himself - so it's about passing on the torch. IMO, it should be something like:
Post P1 he's 'detained' by Dillon's superiors. They've been in operation for some years after happening upon their 'first' encounter (this would flesh out Ana's story a bit) by accident while spying on Cuba during the Cold War (or something els along these lines). It could also touch on things like Nixon's Strategic Defence Initiative in the 80s as a response to ET activity as opposed to the Nuclear Deterrent they fed the public. (serves to show how high reaching OWLF were/are).
Arnie is recruited but with the intention he gets a unit together to investigate/eliminate any future threats. This is agreed to, but at the same time Peter Keyes is on-boarded and the 'science division' have their own ideas. This leads to Keyes being sent to LA in '97 instead of Dutch that unfolding as we know.
Post P2, Harrigan is put through the same interrogation by the Govt./OWLF where he and Dutch bond during his 'stay' over their mutual dislike of Keyes and what he stood for and both remind their 'bosses' that Keyes and his team screwed the pooch big time. Harrigan is not recruited as he just wants to put it behind him and forget it ever happened.
He goes back to LA as a cop but is forever under surveillance by the Govt. and Dutch keeps contact with him outside of his job, as it were.
I don't think Harrigan's character was in any way as strong as Dutch's, and I'd use this as a means to write him out the story, being frank. Plus Danny Glover these days looks like he finally got too old for this shit!. Maybe Harrigan returned and kept his secret for years when his son and wife are killed by the LA gangs he wanted to go back and rid the streets of? So he spirals into a depression and after years of alcohol abuse/breakdowns he kills himself. He has a quiet funeral and laid to rest at Arlington for his 'services'. Could have a scene where Dutch visits his 'Old Friend' and places Danny's necklace or Adolini's pistol at the grave?
The pred encounters continue at various spots throughout the years until modern day, at which point I'd introduce a Bad Blood film whereby a Pred goes nuts, and either a team pf Preds or lone Pred is dispatched to kill/retrieve him. At which point Dutch's team re-enters the fold. (I posted my idea for the next film in the Pred 5 thread so won't take up any more screen space here).
