It is a double edge sword to make racing appealing to fans. On one hand, if the rules are totally open, you'll quickly see one or two teams who dominate and end up leaving the rest of the field in the dust. 99% of that is based on budget, so you'd need to put budgets in place to keep that from happening, which will affect the amount of development and somewhat negate the openness of the series. In some ways, F1 suffers from that. The big money teams are the ones that dominate while the rest of the field is basically finishes in order of the team pocketbook.
To help prevent that, an organization could to start specifying out how the cars will be built. To the point of designing the cars to look the same and, in the case of IndyCar, even providing the chassis. Nascar is the same way. It provides much closer racing, with multiple passes, but at the expense of wild development. The only differences in the cars (that a fan can recognize) are the color schemes.
That said, what do the fans want? I'm not talking the racing fan like many on this forum, but the more casual fan that watches other sports. What would make them watch racing more? For some, it might be seeing a team dominate (as long as it's "their" team), for others, it could be the close racing in a spec series where driver skill and willingness to be aggressive or take chances are more exciting. It could be seeing a breakout athlete. For example, a football player breaking through the line and running it 80 yards is exciting. How do you breing that same excitement to racing? I honestly don't know.
All of that said that, one reason I think Nascar does well are the ovals. While it's easy to point out that all they do is turn left, the cameras are able to keep up with most of the field for the whole race. Looking at things like F1 or IndyCar or IMSA, because they're on road courses most of the time, you only see the full field at the start or at a restart. 90% of the race coverage is on just a few cars at a time. If you're a fan of the one of the back markers or even mid pack teams, you'll see your team for less than 20% of the race.
I don't have a solution, btw, just pointing out why I think it's challenging. It really comes down to getting "normal" people interested. It looks like the background drama (whether real or manufactured) has helped F1, but I'll be curious to see how that pans out in the long run. I'm also curious how many people watch DTS, but don't watch an F1 race. I know my boss is one of them.
-Rob