FifthView
Vala
Oh man, Fifth, I feel you on the plot thing. So much.
For me, I tend to have a very similar problem over and over. I have a character concept and the immediate situation she's in, and then I figure out some stuff to happen, but then I feel like I go off track and just add things, but don't exactly have a great journey to the end? Maybe that's clear?
I think that problem is common for pantsers?
I would love to be able to offer insight, but I'm still feeling my way through my own issues with plot. I have to psychoanalyze myself, and that's not easy.
Like you, I can create characters I like, and even a world (in general) that I like, and from that point I can imagine a great many scenarios or scenes that interest me. But having a coherent plot that interests me as much as characters and world....not so much.
I don't begin writing with only characters and world in mind, but have to have some idea of a plot before I begin; so, that's where the problem starts. Often, I think in terms of what will display my characters and world well, the sort of events that will shed light on the various facets of my characters and world. (I like making characters suffer, for instance–although usually, emotionally and/or existentially, not so much physically.) But problematically, potential events are abundant. The way I approach things means that my main characters will be interesting regardless. (At least, that is my intention.) So how to settle upon a single plot, a coherent thread of events, when just about any will do? Plus, I often feel that no plot is good enough for my characters. (A plot archetype just seems flimsy in comparison to a fully-fleshed character.)
I think my problem, and perhaps yours?, may be that plot needs to be driven at least in part from the outside. From outside the main characters. It's not enough to have just a series of events that allow a character to shine or suffer. I suppose a character-driven approach can work great if the character is a strong protagonist–is protagging strongly, or extremely proactive–but if he's not, or is often reactive (you want to show the facets of his character through reaction to exterior events beyond his control), then having a good plot means letting go a little bit of the character-driven approach and letting something else drive the story forward also.
Now, I don't know whether that means creating a compelling villain to protag (a new character to help drive things) or just building up the world so that it protags in a compelling way. (World in general. Could be physical, e.g., ecological, geological...or could be human forces albeit without a single face or villain behind those forces.)
It's just that I feel that a convincing plot, when the MC isn't a very strong protag, requires the willingness to acknowledge that the MCs are not the end-all, be-all of the story, and that other things beyond them are just as important as they are, and need to be as fully fleshed...if that makes sense. I also think that maybe we always need a great answer to this question: Why are these characters and that exterior reality coming together just now?
I realize this is rather vague and may not be helpful in the slightest.