on the major existing decentralized exchange, Bisq, you already have a publicly observable price feed for XMRBTC. you can calculate XMRUSD based on that and a BTCUSD price feed that you trust.

DEXs that are in the making, namely Serai and Haveno, both plan to have at least one XMR pair with an Ethereum-based dollar-pegged coin. I suppose, although I'm not sure, that at least in the case of Serai the trades on that pair will have publicly observable prices.

Haveno will also have non-blockchain, actual fiat (cash, wire, payment apps) pairs with XMR.

you can take sources like this and calculate an average or a median. current price aggregators, like coin listing sites, will probably do the same, so it's likely that even in a DEX-only future you'll get your prices from the same sources as today.