I think this just nails the problem (or strength, depending on your point of view) regarding Caribdis' main protagonists: they're not really blank slates that act as stand-ins for the player. They have their own personalities (often leaning to the actually-a-hidden-badass, white-knight persona), their own histories, their own desires...so it's kinda hard to write in choices that are out-of-character for them but could provide for some "interesting" directions to the story (and by interesting I mean only for those who's in it for the fap and don't care for initial character development being chucked out of the window like uhh...I dunno...fans of the Taimanin series?).
Not that I'm saying that he should start writing the most bland mcbland protagonist in his next game. He's already good with what he's doing, and I don't want his writing abilities to get flushed down the drain for the sake of making a sandbox-style story work. Instead, considering that we already have 2 games with "nice" protagonists, maybe experiment on having protagonists with wildly varying personalities that differ from them in the next games. For instance, have one game with a protagonist that's purposefully written in an amoral light. One that's willing to do stuff that the protagonists of OiaL and Eternum wouldn't do (i.e. is okay with sharing his love interests to advance his goals, is willing to get his hands dirty (and not in a good way), is okay/doesn't give a shit if he is disliked by people/his love interests so long as he gets stuff done), but whose personal development and journey is just as interesting. Done right, and it could provide a way for Caribdis to experiment on options that would be OOC for his earlier protagonists, while providing a look into the world he's shaping from the eyes of someone who's not in it to get every girl or save the day.
(All of these are just personal thoughts and suggestions from a random forum poster btw, and by no means demands to Caribdis lol. It's his universe, his games, his design choices after all)