I usually pants with a general direction in mind, but I have two characters I'm not sure want to go that way. If they don't, I'm not sure what the story will build toward, if anything. How do I keep it from meandering until I have a swampy river delta?
Let it meander, you never know what will come of it. If you don't feel it works once you finish, there is always the editing process, to fix the problems you see.
Discovery writing is in an inherently messy process. You basically need to explore with your pen (or keyboard!) and just write. If something doesn't work out, you erase it and go back to try again. Or you finish the whole draft and do the editing on the second revision instead, but honestly, it's sometimes hard to build on top of something you don't like. This is exactly why I stopped doing discovery writing for novels. It's way too easy to go down one route and end up in a complete dead end, ponder where you'll go from there, or worse, loose all your motivation and never finish the book. My advice to your problem is to trust your instincts. If you really feel that your characters don't want to go down in one direction, don't take them there. Consider another. In fact, think about what your options are. You really don't want to write 15k+ words and realize that it all needs the select+backspace treatment because what you have already just isn't compatible with any new ideas or direction. Discovery writing is fine if you can handle it. Actually, I really like just getting on with it myself. But that brings way too many problems that I just can't handle.
Part of the problem might be that you already know the direction you want your characters to go or have some sort of end game in mind. I'm a pantser and have pretty much always worked that way. I have also never had issues with plot or things happening. I also never really known when I start writing a direction I want things to go. So, I don't feel a need to force or move things in a certain direction. I think it's just how some writers work. But it seems like you already have some sort of at least loose story in mind. When writers plot out a story, they are just doing some sort of quick run-through of the story before fleshing it out. That is something I always had problems with. For me, it feels like an extra step, basically writing the story before writing the story. And then I don't really feel like writing much because the story is done (in a way), but I still have to do all the work. But, again, this is how some writers work and what works for them. With either approach the rules of story writing don't change. You still need a plot. You still need character development. You still need pacing. And you are still in control. You can make your characters do anything you want any time you want. That's true whether you pants or plot.
Often the hardest writing work I do is done far away from from a keyboard. I go for walks or sit doing some kind of handwork (usually embroidery) that allows my mind to wander and explore possibilities. Sooner or later a workable idea presents itself. It took several months of ratiocination for me to solve who dunnit in my novel, and I was halfway through a long walk with the dogs when the answer finally hit me. The solution was brilliant, if I do say so myself.
I used to walk a lot while writing (write while walking?), but these days I usually either sit outside at night, pace around in the house, or just think while I'm doing whatever monotonous job (it can wreak havoc on cooking! I eat a lot of overcooked eggs and sausages for breakfast). A lot of it now is done at the bus stop, until somebody else shows up and we end up talking. I also find a lot of ideas come up shortly after I wake up, especially if I'm thinking about a dream. That doesn't mean the dream determines the story idea, more like I think through it, what the symbols probably mean, and it usually connects up with my life in some way that makes sense, and then story ideas can come from that.
I pants and I sometimes go back and completely change the story mid way through, in some cases I've split a sub plot out and made it a separate book, or changed who is the point of view character or whatever... if you're in a swamp and meandering around i'd stop and reevaluate what the story is that you want to tell
I am an unintentional pantser. I usually have some outline of where I want a story to go. Then I sit down and my story starts telling itself and there goes my outline...