As the title suggests, it would make the most sense for a fractal based around Scarlet's Attack on Lion's Arch.
We already have the Molten Alliance Bosses Fractal.
We already have the NIghtmare Tower Fractal.
We already have the Aetherblades Fractal.
Now we just need the Battle for Lion's Arch which is already set-up to easily be like a Fractal - We see the destruction of old Lion's Arch, we fight back to take over each section of the city - Molten Alliance, Aetherblades, Toxic Alliance and maybe end it with fighting the giant Scarlet Hologram or even aiding Dragon's Watch in killing Scarlet herself.