ROI comes from the price changes of something has stable value then its not an investment.
if we only keep our stable coins, i think there will be no ROI my friend
but, if we trade this stable coins in BTC or ETH pairs, i think we will get a good ROI mate
how, ? just exchange your stable coins to BTC or eth when the price of BTC or ETH will rise, then exchange your BTC or ETH to stable coins, when the price of BTC or ETH will go down mate, maybe thats the right way to get profit from stable coins