Isabelle shouldn't be a cause for concern for anyone here. Even if she's not an echo character, she honestly makes more sense as a well-known new character to add than others in that mold...Elma, for instance.
While people are discussing rumors, I want to bring attention to what I believe to be a legitimate leak, by a GameFAQs user who has said some pretty interesting things. Start here,
on post 67 in this thread, and read everything else loz18 has to say over the following pages.
Before E3, loz accurately leaked:
--Everyone is here
--OoT Ganondorf with a sword USmash
--Mario getting Cappy
--Much of Inklings' moveset
--Stage selection before characters
--Link having a regular grab and rollable bombs
--Moray Towers
--Parrying after perfect shields
--Spammed dodges becoming slower
He wasn't spotless in that thread, to be fair...he said wavedashing appeared to be back, something we now know likely isn't the case, and said there was a strong indication there would be reveals at the E3 tournament. But I think he's clearly proven access to one or more sources that at the very least had access to the demo.
In that thread, he also comments he's heard strong indications of a Heroes vs. Villains theme, describing that as "something that's been floating around." Which makes me think...remember when the Imgur rumor, full of bizarre character choices like Porky and Goroh as part of a Heroes vs. Villains theme, came out, and multiple well-connected mods and users here and on ResetEra wouldn't immediately dismiss it as garbage (before someone who works at a third party company said he knew some of its newcomers weren't in)? I'm willing to bet they, too, heard about that theme way back when. And if it's legit, I do think it's as close to a K. Rool confirmation as you could feasibly get, unless they just go out of their way to have villains who aren't him.