One idea I have is to make Star-killer Base into a literal base, not the weapon. The weapon is a mobile weapon, IE either the dreadnaught, or Snoke’s ship from 8. The Gun of Star-Killer from seven becomes the dock for the mobile weapon which has the star killer laser on it.

The weapon can track you through light speed, but because the weapon on it is so huge, the base is its primary shield source and also the means to charge the now mobile superlaser.

Having the weapon that destroys the Hosnian System be a mobile weapon differentiates from the older films.