[@Mokley] Hmm...well, the only idea I have currently is the over-used idea of everyone being in the same island/place/town/etc and running into each other. I can see that logically happening but of course, the hard part is how to explain each character's presence there. This will, of course, be easier for some characters but maybe a bit harder for others.
Other than this, I can't really think of another plan for everyone starting together.