Maybe a silly question, but if the concept of a stable coin is to maintain market cap (i.e. coin price) by dynamically altering the supply and/or demand. (e.g. Tether, Ampleforth) then what is the potential for a return on investment? For example, if USDT is always about $1 what's the upside of investing in it if the price never goes up?
Before thinking of ROI you need to be first sure of the coin you are investing in, because some projects can exit scam before they even start, and in such case, of course you have already lost. Now assuming you have bought coins like ETH last year and you are holding, by now you ought to be in profit, thus depending on how much you have invested and how much you already have now, you should be able to know your ROI.
On the other hand, stablecoins are different, as their general name goes, "stablecoins" which means they are stable, and it is in clear sense to show that, any investment in such coins (stablecoins) will not yield anything, no ROI just nothing. The only use case associated with stablecoins is just stability against the crypto volatility, and nothing more. So if you are thinking of a coin which can give you a good ROI, then just know stablecoins are not the right ones, because it is just stagnant, stable.