In fact, unleaded avgas with an octane rating of 100 or more is not too difficult to make. Technical-grade xylene (or an unseparated mix of toluene and xylenes) would have an octane number in the 100-110 range and may even be slightly cheaper than 100LL. Unfortunately, it has a much higher inhalation toxicity and is more aggressive to elastomers than regular avgas, but this isn't an unsurmountable problem.