Alright my two cents on the matter. I personally would be ok with keeping Splash the way it is. Most people probably wouldn't connect Splash to a movie that hasn't seen a wide release in decades. On top of that, Disney's attempt to make Pirates slightly less offensive ended up making the ride slightly worse.
However change is likely to be inevitable so
A Princess and the Frog retheme would be great for Disneyland's version of the ride provided Disney decides not to cut back expenses after a drop in attendance like they're famous for doing.
For MK's version
Keep the ride the same, but change out Bear, Fox and Rabbit for original characters. Keep most of the animatronics and change the score. This is probably the most minimal change of all. Stuff would be modified to fit the new story/characters but most of the ride should still be largely intact.
Give the mountain a total overhaul and change it to a mine/frontier theme similar to Timber Mountain at Knotts. You could easily just remove the grass/green around the mountain and replace it with earth tones. Interior would obviously get a full overhaul with new show scenes and maybe incorporate some modern elements into the ride like projection mapping. Don't cheap out on us Disney.
And lastly, combination of the last two ideas. Give the mountain a full overhaul and use original characters. Completely new show scenes but cute animals are still the focus of the attraction. Brand new score, modern elements mixed with classic show scenes.
This is all banking on Disney not being cheap and forcing budget cuts however.
This also means my beloved Grand Fiesta Tour might be on the chopping block in the future too. I'm kinda sad about that but I guess that was inevitable too. ?