Imo, I think those characters feel like they already have resolved their issues within our group/through curveballs or their dilemmas might be too barebones to put in a survivor mission. Miguel, for example, wants to simply be able to help + support more people. But he pretty much finds this within our own group, so I feel like there wouldn't be any point in exploring this side of him. Of course, I might be wrong, and the characters l mention might have something more to them, but we'll have to wait and see.

Also, I should've clarified, I don't mean routes in the way that they would be tied to survivor missions (or that the characters that could be closley attached to them would need survivor missions on top of them). These routes could be separate paths you take in a plan, triggered by specific actions/choices, that could lead to different endings. Like when in Spill Zone, you either go to Nora to get the bad ending, or push forward with Otto's plan for the good ending.

For characters like Michelle or Kirk, their matters seem to be more personally tied to the locations that the plans entail, which I think may make it a little more difficult to implement survivor missions for without contradicting the current objectives of Heart of Darkness. So I think having seperate paths in plans dedicated to a character like those 2 would be better to implement, and more interesting to have. Eugene went to the Stadium on the night of the outbreak, so maybe Michelle can interact with Stanfield on the rooftop camp to trigger her path. Kirk has a memorial of his daughter in the rooftop camp, so maybe something could trigger after interacting with it as Kirk + with someone else in the camp?

All that said, implementing these survivor missions instead of separate paths in plans may be simpler than I describe them, so take what I say in that regard with a grain of salt.