I like PoS variants with security deposits, like in Decred and perhaps in Ethereum in the future.
In these variants you must lock a part of your coins to receive rewards. That means that for the time you lock you coins, you are backing its value because you cannot sell them.
Proof-of-burn is pretty similar, although a little bit more risky, above all for "small" minters. In PoB you destroy your coins to get rewards, and never won't get them back, but the equation there is that it is very probable that your reward income is greater than the destroyed coin amount. But it has the same "backing" effect, as burnt coins obviously cannot be sold.