Lucien will not return, its been fifty years since his death. He would be over 110 by this time and even if he did, a 110 year old can barely walk yet alone fight and prove a challenge to the ruler of albion.
I wouldn't want Jack to return, as cool as he was, it would be confusing to the ending of TLC.
I have no idea what the creature is, but it will be badass, since an achievement, kept as a secret has been made just for it.
The only ideas I have is a dragon, the ruler of Aurora or something never seen before and I think the third option looks to be the brightest.