Lovely pictures! I think we could use a lot of those.

Fantasy World:

Maybe runes could be used when brewing potions? Like, say you brew a potion that heals your wounds. If you were to use a fire rune on it, then it would not only heal you but give you a boost in energy. Or maybe even make you fireproof for a minute. I'm okay with four basic runes, though I imagine we could mix and match, so to speak. 

Sci-fi World:

Yes, I loved GotG, too! <3

No robots is fine, though I think AI would still be a big part of the world. Hoverboards, aircrafts, and the likes are fun, and I think we would have holographic interfaces too. [url=http://i29.photobucket.com/albums/c260/Genkai001/Settings/27616896.jpg]Like this![/url] 

Modern World:

If the sci-fi world and fantasy world were still contact with each other, what could be the reason the fantasy world hasn't progressed with their technology? Would it be because they have no need for it?

Misc:

I like the idea of having the three different worlds pursue completely different goals. Makes for a lot of conflict. So if we were to go with that: 

Modern World somehow gets the prophecy backwards, and believes that it called for the three worlds to divide instead of to unite. Fantasy World gets only half of the prophecy—the part that warns about something really bad happening—so they prepare for that. They don't know that staying with the others worlds is the only way to stay safe. Then we have the Sci-fi world, the only one that got the whole prophecy down. They've been doing everything they could to reunite the three worlds (maybe that's why they have such advanced technology? They've been trying to build a new method of communication/transportation between the worlds).

Also, any ideas for names yet? :D