1.5 has Sora sitting on the throne, the sky goes from twilight to night.
2.5 has Sora standing and the sky goes from night to day
2.8 has Sora walking and the sky at daybreak
KH3 has all main characters reunited and a combination of all the previous skies.
I've always interpreted it the easy way:
Sora's poses are meant to reference his progression through the series (from being an ignorant child stuck in his homeworld to standing against the darkness and finally moving forward, gaining maturity and meeting many friends along the way).
The sky changes for each cover leading to the KH3 one where all skies collide, these chapters reach their conclusion and the associated characters reunite. It kinda mirrors the continuity of the series. The worlds fall to darkness (1.5, from twilight to night), they are restored and saved (2.5, from night to day) and 2.8 has daybreak sky, both an obvious reference to the age of fairy tales taking place in Daybreak Town and a more subtle metaphor for new day, new story hinting at the connections between KH3 and Union X stuff setting the basis for what's to come and reinforced by the darkling in the cover art of KH3.
But now that Oracle mentions it, I can't believe I didn't realize the similarities between the covers of the KH collections and the vs XIII/XV promo art... that was totally hinting Yozora.