I have some opinions re: a hub concept. I'm not planning on making any levels here, so this is just advice.

First, use only one hub (or at least, don't add an extra one unless it's absolutely necessary). This is important, moreso for organization and gameplay than for technical reasons.

Second, don't make the hub at the start of the project. Make it last. Just have a vague idea - there are some good ones in this thread already. Just say "ok, our levels will be connected via this space station/city hub level" and then focus all efforts on the actual *main* levels.

Once the main levels are done, THEN make the hub level to connect them all. The fact is, not everyone will finish their levels. In fact, in an open project like this, I'd say you will be lucky if half the people finish their levels. Thus, you make the hub based around what is actually done and NOT what might be finished. Really, it should be one of the last levels made!

It's also important to note, _designing_ a hub level is one of the hardest things to do in jcs! In The Fortress of Ruin I used a sort of hub level (devan's fortress), although the pack was linear. But still, I had to design a level in mind that would be revisited continuously, open up gradually as you revisit, be open ended, and not fall apart into an unplayable mess. The only way I was able to design this level was to know, before even starting it, exactly how many levels it would connect to, how it would connect to them, and what those levels were.