Writing Science Fiction

I'm writing a Sci-Fi story set in the year 2190. It's been so much fun to work on because, really, almost anything goes.

Right now, the story is quite technology-driven. In building my world, I'm constantly asking myself how today's technology might evolve throughout all those years. What will cars be like? What will fashions be like? Etc.

So when I get an idea of, say, the transportation system, I have to make my characters go somewhere so I can write about it. In order to make it really interesting, though, they have to be going somewhere important. It has to move the story forward. As I'm writing, I know my critique group is going to nail me if they're just going on a joyride for no real reason.

So far, between the basic story idea I have, the rough synopsis I was forced to write last year, and the technology I've been able to come up with, I've managed to push the story through to a certain point. But I keep getting stuck. What's going to happen next? How exactly is this all going to work out?

So, at LTUE this past weekend, I got some more ideas, and I'm fleshing more story out with even more detail and cool extrapolation of what the world will be like so far in the future. Unfortunately, I still don't really know what's supposed to /happen./ This has always been my biggest problem in writing.

Any ideas? :o)