Gula et Mortis

Vander considered himself a "Rabid beast" and so the shimmer turned him into a huge snarling brute.

Jinx's childish and playful, so it made her even more excitable and fast.

Viktor wished to improve his own body, and that is what happened, mixing it with hextech.

And while not in the show, Dr. Mundo clearly is pumped full of shimmer, and he was a 'kind man, who acted foolishly' and it turned him into a bumbling idiot who thinks he's doing good.

Here's my theory.

Shimmer is Void based, and we know that the Void basically deletes reality.

2. Singed used what we could presume was refined shimmer on Jinx during a long drawn out operation. We know this stuff in small doses already makes someone faster and stronger like it does with Sevika, so extended use during the operation likely had a lasting effect on her body.

3. Viktor was using a MAGIC HEXCORE with shimmer. I think thats the reason it might have fused with him. Not really sure where “its what he wished” came from.