For me, the most interesting question is - how did Indra, Mitra et al fade out and Vishnu, Shiva/Rudra and Shakti come up on top post-rgveda? Plus, Skanda/Kartikeya, Ganesha. The epics and puranas that emerged after Rgveda seem to have fashioned this transformation. The why of it is difficult to understand. The How of it may help. Keep writing this series !