Depends. On the one hand, I want us to do something like DnD's alignment style, but on the other hand, I don't want this to be a rip-off of Shin Megami Tensei, which this could even accidentally fall into.
I'll have to vote for character-based. I mean, we could easily make characters to fill the gaps for each ending, but it would have to be pre-approved before we got to work on it. For example, I was thinking each path would involve one of the founding families of the Grail War and a family that originally had nothing to do with the Grail War in canon. Each end would be because of a romance between the MC and a female from each of the families.
Of course, if you guys don't like the idea, you can overrule me if needed.