CGM won't have much romance, and since Chandragupta married the daughter of Selukus Nikator, it's would be unrealistic to show romantic scenes b/w them like, say, Avdesh & Arpi. So whoever plays Helen should be someone who could fit that role, and I don't think it's Tanvi.
Tanvi could go to Dwarikadeesh as maybe Draupadi, or Radha (if they show that part of Krishna's life) or Jambavati. In fact, she'd have made a better Rukmini than Priya Bhatija-Shah.
If she doesn't go there, maybe Nivedita could go there, and play Gandhari đŸ˜ˆ