One idea that just came to my mind, even though I don't really like it:
Allow Anubis to 'trigger immortality on/off', eg. he would be the only unit able to target immortal creatures, and doing so would remove immortality from targeted unit.
This would make him a kind of counter against immortality used against you... and would allow for combos with 2 Anubises on your side (eg. deimmortalize your creature, buff it, immortalize it with the second anubis).
I wouldn't mind if the cost of ability was increased to 2 or 3
in such a case. (would synergize with SoR then)
Though, as I said, I don't like this idea of mine. Mostly, because for me immortality is a permanent state that means the creature will never ever die. Of course fire buckler/thorn carapace prove this definition wrong anyway, so if someone likes this idea, here it is.