neither of those!
What is wrong with just Planets? The universe is endless. You dont need planes, nor dimensions when there is deep space. It is way more terrifying, to know that IT is fisically out there. (Whatever "IT" may be)
A spaceship floats out of control and sends an SOS-code.
As the heroes aproach and board the dead, floating ship, they find no sign of humanoid life. Just a weird fungus and near tropical temperatures in the ship.
Where is the crew and what is the secret of the fungus?
What actually happened:
the air filters broke. From the moisture of breathing and dead skin cells, originated a fungus (many years into the deepspace Mission). Witch grew and came alive after absorbing enough radiaition from strange metal samples (brought from the deepspace Mission).
The Crew was absorbed into the fungus, that became a hive mind of everyone it absorbed.
Now it is hungry and lurred the heroes into its trap, to absorb them too.
Mission end? Kill it with fire and take the strange radiating metal, without mutating.
I don`t want to see mighty magic. It should be the era of technology! Maybe magic as mysticism of some sort. But that should be the maximum dose of it.
Don`t make it too similar, to Golarion-Pathfinder (witch i love)