Try to use sesame oil for the tempering, the flavor is out of this world.
Any bland juicy vegetables would work for kuzhambu type gravies. My favorites are carrot, green pepper, yellow pumpkin, broccoli stem, chow chow (chayote squash)!
This kuzhambu reminds me of my patti (my mom's mother). My earliest memory of this kuzhambu is eating it at her house, what we called perambur patti house (Perambur in Chennai where they lived, as opposed to Purasawalkam patti, where my dad's parents lived). This kuzhambu, followed by thick fresh curd, slightly reddish from the clay pot in which it was set is a nostalgic meal I try to recreate, minus the clay pot yogurt, when I'm struck by pangs of nostalgia and craving comfort food.