Maybe IndyCar should introduce the Hanford Device, which was used by CART in the late '90s and early 2000s. Its was designed to increase drag, in order to reduce speeds on superspeedways and increase the draft, giving the car behind more of a tow.
This clip of the last few laps of the 2000 Michigan 500, is a good example of what the Hanford Device can do.
https://youtu.be/25bHlzCbLuA
Alternatively, bring these cars back. Just listen to those engines.