Gates of Gre'thor is an intruiging mission for the Klingons, but I have a few suggestions. Aldra in "Devil's Due" (TNG) claimed to be Fek'Lhr(I use an L instead of capital i to make a distinction from the species). While the character was a sham, the fact that she used the Orthodox Renisance Christian version of the Devil durning the court room, it could be reasonably argued that her version of Fek'Lhr was accurate depiction drawn from Klingon texts and art. I reccomend that that version replace the Fek'Lhr version in this mission. Or, better yet, have Aldra's Fek'Lhr as a General ranked monster you fight in First City. Maybe make it a super fight with all of the major NPCs joining in for the grand melee.
On another note, Molar was too easy. After getting past the mob in the antechamber, him solo was anticlimatic. I suggest that the sisters of Duras, Lursa and Be'tor make an apperance in the game as Molar's "succubus handmaidens". Having two missions in direct combat supporting him would make for a more challenging fight. They would show up once he is aggroed, similar to the borg spawns in "The Cure". Only when Molar is down for the count, do they stop respawning.