You're right I didn't mention specifics of on who would create these. A naive answer would have been "why not anyone?", but you bring up an interesting point on who would actually hold the bet for a long duration of time and be guaranteed for pay-offs.
I disagree with the problem of enforceability of the paying out in case someone looses a long position, since it's also a current problem with the standard futures for stocks/commodities, where the brokerage firm would be liable for paying up in case of defaulting on a margin call, so similar structure might be imagined for these cases.
With regards who might create these futures, the government seems like a good candidate, but really if we do have a solid clearinghouse with similar structure for handling margin calls etc. I don't see why not anyone can create these.
The regulatory concerns for implementing these is very valid, but I was primarily just interested in exploring the idea.