It's hard to say off the top of my head, but I think having stablecoin options is important, ETH, BTC (harder to do with bitcoin wallets). price should be configurable by the seller, so it's just a simple buy button for the vender. most of the POS crypto stuff I've seen is very overbloated, I think the whole thing could be simplified to an iframe button that opens metamask. This is sort of an idea I've been toying with for some time and built a prototype some time back. that's my 2 cents