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IIRC, Kalshi has this (aspirationally) on their roadmap, but I think the regulatory hurdles are even more severe than for $-denominated prediction markets.
Hmm, I wonder how long the term and how thin the spread must be for this to be financially equivalent to just betting the S&P500. I think it's a promising idea to take away the first-level objection, but I'd love to hear from actual financial analysts or traders about when it's enough to get them to invest/bet.
The fundamental problem of long-dated transactions having either large counterparty risk or locked-up assets (which ALWAYS have some sort of option/liquidity value that's given up) isn't solvable, but this may make it somewhat better.