MakerDAO uses collateralized smart contracts to provide stability to their Dai token. Right now, only ETH can be used as collateral, but in Q2 they are starting multi-collateral support.
Their whitepaper's explanation of how this works is horrendously complicated. There's no way you can encode that sort of logic into a cryptocoin in a bug-free manner. Steer clear of this.
And who will abitrate the collateral? While it's smart wallets and crypto currency it can be enforced by cryptographic proof but when the collateral is USD then it's going to have to exist somewhere.