In my mind Bisq with full lightning integration would split trades into tiny payments so that the security deposit could be re-used for each and thus keep the security incentives while reducing friction.

On the Bitcoin-to-fiat side this would take advantage of some API-enabled instant payment solution. In Europe the regulatory roadmap forces banks to:
 1. Open their APIs to 3rd parties
 2. Implement SEPA Instant payments

So Bitcoin-fiat micropayment powered, lightning-network enabled exchange should be very possible in 2019/2020.

...but if you don't want to wait that long, combining  ⚡️lightning network ⚡️ with a pegged coloured coin (like USDT or similar) would allow very fast atomic swaps.