We don't have CART era budgets so CART era cars won't seem to fit.
They need to do something creative. Actualy creative, not just small steps of the current format. Current Indycar doesn't work, and unlike NASCAR or F1 they really don't have a lot to lose.
They either need to
-Tender out the engines as a super high tech/forward thinking live R&D exercise. Ie car companies are going to spend *silly* amounts of money on future technology. Why not give them the chance to spend that money *and* do some marketing. The upside of that is if Ford is sponsoring you as advertising, you can lose that money to the NFL, TV advertising, whatever. R&D money isn't going to be poached by Major League Soccer.
-Go back to basics. A really basic/cheap car(with the requisite driver safety) but then open up the rules a little more so there's some diversity. The fans aren't as tech savvy as they think they are, they just need to *look* different. "Innovating" is a mostly visual thing. This will mean the end of higher tech engines probably. They need a huge 700-750hp engine that's accessible in cost/technology. And then design car rules around that so 750hp is enough to light up the rear tires on road courses and do really high speeds on ovals(on the straights). The corner speeds may need to come down, but that's manageable. Would you take 250mph down the straights at Indy and a 200mph corner speed? I would. You still get a headline speed figure. I'd be the first one to buy a qualifying day ticket in Turn 1 because I want to see how deep the bravest guys are going before lifting, and introducing some slowing down for the turns/handling will make the race better/more interesting.