Now that The Wicker Man is awakenable I think that the next old character to get an upgrade (either special cosmic, awakenable or variant) will be Goddess Aset as, unless I've forgotten someone, she is now the only boss character from the original story line that hasn't been upgraded in some way:

The Wicker Man -> Awakenable
The Nomad -> Special Cosmic
Goddess Aset -> ???
Horus -> Awakenable
Alliance General -> Special Cosmic
Corrupt General -> Feldmarschall Variant
Beelzebub -> Demon Prince Variant
The Prisoner -> Awakenable
Wrath -> Demon Prince Variant
The Beast -> Awakenable

The other option would be to upgrade on the of the Killers (Killer/Killer Prime/Killer Beast) but I think these may still be a little new for an upgrade...

My theory is that once all the original bosses have been upgraded we will then see them (max awakened, with cosmics or as variants) in an Insanity level mode for all original story missions. The addition of an Insanity level to the Frozen Wasteland dungeon with max awakened Beast plus both Demon Princes (see update post here) seems to support this idea.

What do people think? Could Aset have an upgrade coming to then boost all the original bosses for an Insanity mode?