Elise can be a vastaya. Rengar too. Wukong. Ahri.
All lores can be scrapped so that every animal-themed character is a vastaya, aka the "generic anime furry class" of every mmorpg ever.
Somehow, Nidalee doesn't feel so special anymore. She's just one of an entire race of people who can turn into animals.
Unless they change her lore Elise isn't a vastaya. Her powers were granted to her, not something she was born with.
On to your main point though I think it was a necessary sacrifice. Creating something solid that helps build up Runeterra is important, and I believe Nidalee is still the only Vastaya with shapeshifting powers that we have, so she still preserves a unique element.
Elise is not one. It's already clear in her lore that she's more of a magical mutation after a visit to the Shadow Isles.
Nidalee probably isn't one either because she's not a mix like others if this race. She is fully human or fully cougar depending on her transformation.
well I guess darius garen and jarvan aren't special either because their human.
To actually get a better view of this take a look at the void. While a lot of the characters from the void have similar attributes they are still different in many ways. Just because there is an entire city of them doesn't mean they cant all have aspects that apply to each individual from the story it sounds like there are many different groups of vastaya that we still don't know about. plus if nidalee lore is changed to support this she will still be fairly different from the rest of them as she can still transform which even among the vastayans would still be rare or at least mildly uncommon.
