What do GOOs want?

[Cheesegear]

You can have the GOOs have an active care with the Warlock. In this case, perhaps they wish to wake fully from their slumber, in which case the warlock tries to raise their patron fully.

You could have the warlock having discovered an inkling of terrible knowledge that imbedded a sliver of the GOO's power into their soul. The GOO doesn't notice, but the warlock now cannot sleep soundly as they are haunted by nightmares and now have strange abilities that grow as they use them. (perhaps if this warlock dies without being resurrected, their soul is dragged to the GOO and is consumed).

To learn when the stars will be right for them to fully enter the world and spread madness. Steal books of astronomy and astronomical items.

I like the idea of combining these two.
As he's gained the knowledge of what the GOO is, and taken a portion of its power. He now realises the threat it poses, and needs to acquire star charts and astral and celestial rituals to prevent the GOO from entering the Material Plane.

This GOO warlock would not be evil or against the deities of 5e specifically, maybe even an ally, but their goals could be the same.

I'll pass this on.
Deities or servants of the same, did a thing to the PC, and the PC went looking for a Patron and/or knowledge to get...Closure and/or Revenge on what took place. The Patron he found wasn't a Demon, and wasn't even an opposing Deity, it was something else entirely, and what has been seen cannot be unseen.

(ie. you can give them contradictory and strange suggestions about what their patron wants based on their interpretations of dreams and visions they get)

My players frequently catch me out when I say something in one session that contradicts something from a session or two ago.

I can now do that on purpose, and in character. I love it.

It looks like some warlocks just steal power from the GOO but the GOO are to powerful to notice.

That's the 'Get Out of Roleplaying Free' Card, and I wont let that card be played.