While I'm at least a mostimalist if not a full-blown maximalist, I think people will want to see stablecoins on lightning, which essentially means a DEX based in bitcoin. The major driver in this will be the user experience. A key component in the user experience is how much prior knowledge is required and unfortunately that may mean that people will use fiat over the lightning network before they are comfortable with bitcoin itself.

Another driver is a mature market for the time-value of bitcoin. How much can people earn as yield by keeping their savings in bitcoin and using it on the lightning network? With negative interest rates in Europe and the developed world, I don't see people wanting to keep their funds in depreciating fiat assets with negative interest rates.