I like the idea that it is able to travel between worlds, and if it were me, the next time that they attempted any sort of arcana check on the ruby to try to use it, or figure out exactly what it does, I'd subletly slip them into an alternate dimension.
Perhaps if you do put them in an alternate dimension, the king may still be alive, the seemingly chaotic neutral pirate may be a royal privateer, the thief may end up as the son of a royal guard and the bard may end up a deaf-mute monk. Each would have their own new backstory, and depending on exactly what happens, they may or may not meet their alternate-selves. To be sure of one thing however, I wouldn't transport them by themselves (send along someone tremendously evil in both worlds), and have the ruby have lost its power after the trip there, with their only return being through the other ruby controlled by the king. Thus they must steal or be diplomatic with the king, which may be hard depending on exactly what you have their alternates do in this world. They'd also be responsible for whatever evil they may have brought with them, which would no doubt also seek the ruby as well as a means of travelling back and forth between worlds, and controlling the power of each.
That's just where I would begin, hope it helps you