This is just way too cool.
As an option, and I know I've brought up Primeval Thule before, but, they use the other planets as the home of demons and any other extra-planar goodies. Instead of demons and devils being from other planes, they're just aliens from another planet. Also works for aberrations too. Just to borrow from a very Lovecraftian approach to critters.
To give you an idea, science works in this universe just as it does in our own, the same laws apply, but magic also works. In the future Earth side of the gate, magic doesn't work except for gods. Venus's avatar can fly through the gate and teleport herself to "Future Earth" where technology works but magic does not work for mortals. She brings her own magic with her, in fact Venus created the gate in the first place. Now what would be her motivation for doing so? I got one, a god's powers are directly proportional to the number of worshippers he or she has. Venus has managed to stay active because she has a planet with a bunch of worshippers on it, but those worshippers number a few million only, it is a very primitive world. Mars on the other hand is more technologically advanced, and despite its smaller size, this technology has allowed Mars to have a higher population than Venus, and thus the God Mars is growing more powerful in relation to Venus.
Now on fantasy Earth, the God of the Christians, Jews, and Muslims is supplanting the worship of pagan deities like herself, but Venus has discovered another Earth through her extra-planar travels, this one is populated by 8 billion humans, magic does not work for them, and they are not so religious as the people on fantasy Earth, and much more importantly Technological Earth has a much higher population that the Medieval fantasy Earth, if she can bring some over to her planet, and get some of them to worship her, she would grow more powerful too, but first she would have to lure some of those other Earth humans over to her planet, perhaps she could give them some sort of mystery to investigate, and atmospheric anomaly, yes, that will work! Unsuspecting humans from Earth get sucked through the gate to her planet, and they discover a habitable planet with natives capable of using magic, and more importantly they can use it themselves if they care to learn it! They would also bring their technology, and that could be helpful too! As for the native cultures on her planet, she reasons that they'll adapt. Earth humans will come in at only a trickle at first, as interplanetary travel is still expensive, but costs will come down over time.