I'm not sure what needs to be said here. I believe it's possible -- and necessary -- for Ravanna to appear in the prequel.
While you may be thinking that since Shiver is an "ancient evil," and since the Wolf Age isn't really that ancient in the Myth universe history, Ravanna must seriously predate the coming of Connacht.
But there are clues in the Myth backstory that suggest Shiver is a kind of eternal evil spirit whose latest incarnation was wrought when Soulblighter brought it into the body of Ravanna.
Hopefully we'll learn how the ancient and eternal spirit of Shiver hooked up with Ravanna in the first place, and what role Damas/Soulblighter and Connacht/Balor played in her being Taken.
Of course we expect vanity to enter the picture... :)
