I am only brainstorming here, but this would be an interesting experiment IMO: what if, for 2020, IndyCar kept everything exactly as-is, but did something in the neighborhood of a 3.0L naturally aspirated V8 for the engine? With the delete of the turbos, that kind of engine I would imagine is close to fitting in the existing chassis.
I'm wondering what the fan reaction would be. My assumption is it would be quite positive. The thought of seeing open wheelers literally screaming around the track again would be pretty nice, and no one is looking at IndyCar as being some standard bearer for automotive tech anyways, so why not just do it?