April Fool's Suggestion Skin: 'Definitely Rengar - Riven Skin'

Typically it would be a 'definitely not blitzcrank' kind of skin, where they're dressed up funky in a manner that belays they are not the champion that they are by bad dressing up (fake glasses + nose on blitzcrank).

This skin would be Riven, except wearing a suit and headpiece that looks vaguely like Rengar (with a button-eye or something similar for comedy, suggesting it was 'stitched together') kinda like the Urf the Nami-tee skin (suitwise design style).

Ult form, the eyes would glow like Rengar's do, her blade would visually resemble a combination of her regular blade and Rengar's claws adorned on it, glowing yellow when reforged. The ult animation would shoot out rengar-claws rather than wind waves.

Give her Rengar's Armor, and just make it look somewhat ridiculous.

Alternatively a Headhunter Riven might fit this similar-theme and style, I just think it would be funny to see an imitation of Rengar skin running around on Riven, the perfect choice vs a Rengar.

If you were to go all out, you can even change her voice so that it sounds like she is trying to imitate Rengar's voice, and saying some of his quotes along the way.