How about just making the Dirge a buildable barbarian UU?
I like giving the Dirge a spell that summons multiple Skeletons with the Boarding promotion. I also make Boarding include the effects of Amphibious, and make it purchasable with Amphibious, Combat V, and Military Strategy.
I used to think that making Margalard, Gurid, and Leviathan buildable was agood idea too, but now I think I prefer spawning them through Nature's Revolt.
I still think that Mary Morbus should be buildable with an Alchemy Lab, Body Mana, and Evil Alignment.
I just got to thinking that it might be interesting for the event that notifies you about the Broken Sepulcher spawned Barbatos there, possibly not as an immobile unit. I don't think this event happens every game (edit: I just checked, and found it is active in only 20% of games) that the unique improvement is there and it doesn't happen at the beginning of the game like the old barbatos spawning did, so it should give civilizations time to prepare. Your explorers finding the stone broken open may indicate that Barbatos just escaped.