I always have ideas, but it seems if I'm going into this will all the answers, I might just be short changing the other guy. Thanks for your input, though. Good to know what has worked for others.
I've never co-written a novel--started to but we never got past the plotting stages--but I have read two of them:
GOOD OMENS was a collaboration between Neil Gaiman and Terry Pratchett. I loved it to pieces but it is the only novel by either of them that I have managed to finish (I read the SANDMAN comics, but that's a separate medium). I've heard different stories as to how it happened, but my friend claims it happened because one of them went "I have an idea for a story but I don't know how it ends" and the other went "I don't know how it ends, but I know the next piece" and so on and so forth until it was done. This worked out well because they had dynamic styles that worked with each other to bring out the best and downplay the worst of each of them. Another friend there during this said "Pratchett kept Gaiman from being too dark, Gaiman kept Pratchett from being too frivolous." (Again, I've never finished anything else by either of them, so I can't say whether I agree with the particular assessment of their weak points. I just know that the two of them together made something that I enjoyed more than anything I've seen by each of them separate.)
ANOTHER SERIES, a YA series of retellings of famous classic stories and fairy tales, starting with Doctor Faustus. It was written by a brother and sister, who came to my college and talked about the making of the book. Because they were siblings and all too familiar with clashing with each other, they did things very differently. Before putting a word on the page, they had every scene and every page outlined thoroughly. They knew what would happen, what the tone would be, etc. All so that they could write the book without fighting over it.
My attempts to co-write were done because I found someone with a similar point of view and we tried to make a story using our combined ideas. It always failed. In one case, I basically gave up on everything involved with my part of the project because I wanted nothing more to do with that person, ever. (To be clear, the co-writing was not actually why I cut that person out of my life. Rather, wanting to cut that person out of my life was why I gave up the co-writing, and all the characters I'd built for it.) And in both those cases, I was working with someone I knew. I can't say I'd be too eager to take on a stranger whose style might or might not work with my own, without knowing if we'd even be able to come up with an idea together. I know that's blunt, but that's my reluctance when I see your post.
Maybe your best bet is to start the way GOOD OMENS allegedly did. Find a seed of an idea that you've always wanted to write, but haven't known what to do with. Post with that and see who has the next piece. That way you're not coming into it looking like you know everything and can create the kind of partnership you seem to be describing, but other people will have an idea if this will lead to a project they'd want to be part of.