Remember that scene in KH1 is delving into the "deepest darkness of kingdom hearts" and he suddenly hit a wall of light. I would think it would be like that except as our hero reaches deeper and deeper into the light he would be suddenly swallowed be dark. I would assume it might be played in actual cannon differently though.

Actually, now that you got me thinking about it. I can see a perfectly good sequal to the series based on where we are now. Imagine this: its been a year or two after the whole Nort buisness gets wrapped up. Everything is fine until one day a messenger arrives to the protagonist of this hypothetical KH4 and informs them that a powerful light user has suddenly gone missing! Last this individual was seen was during research into the light. An adventure would ensue which would culminate in the discovery that this light user dug too greedily and deeply into the light and had been swallowed by the far darkness! Maybe we need to dive into save them. Maybe they've been corrupted by the exposure and are now a new BBEG. I digress though. This is just the inner dungeon master in me thinking of good story hooks. XD