More than you'd ever care to learn about octane improving blending components:
https://www.nrel.gov/docs/fy00osti/28193.pdf
Another fun option would be water injection.
I haven't seen it promoted much lately. but theoretically should be helpful.
EDIT: It looks like water injection systems have come a long way from when they just used to use a windshield washer pump:
https://www.summitracing.com/search/...ection-systems
Honda & MIT had a pretty cool SAE paper a few years ago. They'd get regulsr E10 gas and then split the base gasoline and ethanol with a special membrane. Then during high manifold pressure conditions, you'd have extra ethanol to inject. Not commercialized anywhere as far as I know.