Hmm. Why wouldn't the Lucifer be able to fulfill the role the SD Ravana does in the FS2 story?
It really all depends on the Lucifer's shields.
If they truly are invulnerable to everything (except in subspace), including beams, EMP's, meson bombs..., then the Lucifer is a much bigger danger than the Ravana. Yes, you could destroy it in subspace, but that would require it to leave the nebula system, as intrasystem jumps are nearly instantaneous. As long as it stays in the nebula, it's safe.
If the shields are only resilient (or not at all) to beams, then you have something slightly more dangerous than the Ravana, but then the player can't really participate in its destruction. You have to destroy it with beamed ships.
If there are no shields, then the Lucifer is not really a danger. Yes, it has 8x the hull strength of the Ravana, but offensively it doesn't really compare. Bombers can just kill the two beams and it's done (that would make Bearbaiting much easier).