I would like to see more variety and less restrictions, so I think that something with the following rules would be simple but interesting...
1. Max horsepower limit,
2. Minimum power unit weight.
Otherwise race any configuration you think is the best, whether it be na, turbo, super, rotary, electric, hybrid, 2 stroke up to 6 stroke or whatever. The only other rules would be to do with cost saving such as max rpm or use of certain materials. And innovation is applauded rather than banned.
What would be some other interesting ideas?