Do "Disney" characters belong in Pirates of the Caribbean? Haunted Mansion? Country Bear? Carousel of Progress? Jungle Cruise? Tiki Room (oops...)?
The answer is no. Those rides work great just as they are with original stories/characters, which fit the theme of each respective attraction much better than lazily sticking Mickey Mouse in to appease the people who have short attention spans.
Rivers of Light is no different than any of those. It is meant to showcase an original story and original characters and evoke the same type of feeling that a show similar to Illuminations gives you.
Sticking in characters from Lion King, Nemo, Jungle Book, etc will do nothing to enhance this show if the show itself is no good as is. All you'd be doing is putting lipstick on a pig.
So Jack Sparrow doesn't belong in pirates? Because that's what we're talking about here, not something random like Mickey Mouse.
I completely agree characters will not make a bad show good. But if I was a big Simba fan (I'm not) and I'm watching animals projected on water at Disney's animal kingdom, and I see a real lion image morph into Simba or vice verses, I might get a little more excited than with just the generic lion.
Why do we need Tinkerbell at Cinderella's castle?
Why do all kinds of characters that would never meet all get together for stage shows, villain shows, Fantasmic, Once Upon A Time?
I think at some point a little fun is more important than absolute perfection - which is something to strive for, but has never been (despite nostalgia) and never will be, and is unrealistic.
I've mentioned this before, but I own a record store. I can put Pat Benatar's jazz album in the jazz section where it will rarely be appreciated, or I can know my audience and put it in the 80s section with the other Benatar. If I have more than one, I'll put one in each. There are purists who would (and did) argue with me about that (really!) and there is no perfectly correct answer. But bottom line, it's a business, not a museum, and I have to put it where I think it is more likely to sell, and have employees to help as needed.
We can have fantastic ideas that are curbed by physical or financial limitations, customer expectations (realistic and otherwise) etc.
You should, and I do, always have high standards. But chasing perfection is not a goal unto itself, and can be counterproductive.
And taking theming so seriously at the expense of general entertainment can be also.