I used to be a fervent seeker of intellectually stimulating "general" forums/communities too. I've found a few and two things are apparent to me because of the experiences:
- without specific focus, you get overrun by people that desire "general" intellectual discussion because it's easier to be general than it is specific and generality always dilutes the culture of a group by introducing subjects that not all are interested in (contrast that with specificity, where a specific focus is shared as a common interest between the participants)
- without specific focus, one is arbitrarily created by the."popular" contributors and shuts out any one (by creating a point of focus) that isn't in sync or interested.
My advice: figure out what specifically it is that you want to have a dialectic exchange about and find a community for it. Any sort of off topic discussion will occur in their cafe or off topic areas where the same intellectuals will do some cognitive "offload" by talking about other things that are interesting.
Philosophy, physics, mathematics, programming, DIY, electronics, space advocacy, psychology, etc... are all subject areas I can think of that have many (stimulating) communities built to serve the sole purpose of exploring and developing ideas and thought for the subject matter.
Get specific!