We need a villain celestial being
Pantheon is not villain, even he is not a totally good guy, he isn't that evil either. He commanded Aurelion Sol to close huge void rift which is a good thing imo. Even without it, even if he is super evil inside, he is not that powerful and can't be a huge problem. He can't be a cosmic threat.
Bard is cosmic protector. Aurelion Sol is cosmic creator.
What about a cosmic destroyer?
Both are in neutral - good way. Both are huge anchors for universe. And we know Aurelion Sol fought against a lot celestial beings and in the few of them must be in the same level with Aurelion Sol. I'm pretty sure there are entities he was glad to fight with them. Not all celestial beings can be in neutral - good way. There must be always a dark side.
Someone like this, may be?
http://40.media.tumblr.com/8c2bccfd00c0ece103dd68b442ce2aec/tumblr_nrr0n7EyAO1tjrwu3o3_r1_1280.jpg
Veigar
"The watchers will return"
- Protector
- Destroyer
's daddy
- Death (I think Kindred is leaning more towards the cycle of death and life as compared to the Void)
The
(Voidborn) - Entropy (since they just consume without giving anything back)
or lifeforce manipulator
who created sentient life and habitable planets for Sols stars to shine on.
?