The Firefly is the closest thing to an LS/Twisty replacement. Yes there are issues with the two-level switch, but the stock LS never had two levels, so you can set up an FF the same way. As for greenies, the FF gives you much more flexibility to choose an LED. The FF is also smaller. But I think the FF never reached the LS's standard of toughness.
I guess another possibility is the Surefire KL1 with the Vitalgear 1x123 twisty tail. I'm not sure how that compares in size with an Arc LS. The original KL1's got a bad rap because of the poor LED's that were available back then, but recent KL1's are pretty nice.
Finally I believe there's some hope of improved FF's and FF-like lights on the way, that will make the old stuff seem pretty quaint.
I certainly will miss the LS, but I'll miss the AAA more. It's a shame that Arc went under just at the moment that the L92 lithium AAA battery turned production AAA Luxeon lights into a feasible product that met Arc's runtime requirements. Maybe Peak will do it, but I don't like their form factor nearly as well as Arc's, and Peak doesn't seem nearly as good at electronics.