I think the Culture really only needed one more book, but he'd failed to write it twice: how does the Culture resolve a conflict with a roughly equivalent civilization (two of his last three Culture books tried to address this and kind of lost the plot)?

We see the Culture dealing with much weaker civilizations in many stories, notably Use of Weapons. Consider Phlebas, Player of Games, and others are about the Culture dealing with significantly weaker civilizations. Excession is about a far superior civilization.

Anyway, I'm still very sad he's gone.