1. Agasthya: Parth Samthaan: I know Zain is everyone's favourite here still I feel that Parth Samthaan has that diabolical, suave charm. If anyone has seen him in Kaisi Yeh Yariyaan or his web series Main Hero bol raha hoon, he was amazing as a wicked monster. Moreover with Ayaz being his right hand man, the nostalgic factor would've pulled fans. Vikaas Mantakla too had that sinister aura.
2. Paakhi: Helly Shah or Pranati Rathor or Anchal Sahu or Tejaswi: These actresses have that naive, innocent, dreamy look to them yet they also look great in a grey shaded avatar. Reem has more of that savvy, street smart, mischievous vibe that's not sitting well with innocent, sheltered Pakhi.
3. Ishaan: Though frankly, I'm liking only Akshit's portrayal in the show but if they're planning to make Ishaan the mastermind or even show a vengeful Ishaan, I don't know if Akshit would justify the grey shaded role. Akshit has kind of a likable aura of a beta male which would've worked if they had let Agasthya and Pakhi be the centre stage and Agasthya be a supportive friend or fiancee. But he doesn't feel like an iconic villain. So someone like Shakti Arora or Rithwik Arora could be a better Ishaan.
And the less is said about the supporting characters the better it is. Even if you delete all supporting characters from the show except Yug, the show would be the same.