Ideas for a revival.

I read somewhere in these threads about parallel worlds (I don't have time to read everything). This idea would be even cooler than introducing a third level to the map! It would be really great if at least two maps existed in parallel (and it would be even better if there was a way to generate them), and you could travel between them through some kind of portal! But maybe that's even harder to do than creating a third level for the map. Or is it not? Can anyone answer?