Besides the already mentioned fact about decentralized stablecoins being a thing your idea has an issue

If you back the stable coin with assets whose prices are floating respect to the thing you stablecoin is pegged to, you are left with two options:

Either you simply hold the backing assets or you trade them against the currency your coin is pegged to and among themselves in order to minimize the effects of volatility

In both cases you will end with either profits or loses, and it will make your coin less an stablecoin and more a security. I actually think about DAI as a very well managed security with no immediately apparent way for the investors to take profits or loss, still like it more than other 1:1 cryptos, btw