Chance that the next Gemstone skin is for Aurelion Sol OR Yorick?

They wouldn't give a gemstone skin to a champion with barely any skins.

Assuming you mean hextech gemstones.

Highly unlikely

Neither really has the player base to make a large number of players try and get it.

My guess would be more along the lines of Ahri, Caitlyn, Janna, Syndra or Fiora.

Honestly that would suck for a champ as well who has been waiting YEARS to get a new skin to be locked out like that.