Hello again, I'm the hollow-world user
So here goes :
The planet is hollow and in the middle of it (where the planet core should be), there's a massive energy source, which serves as the main source of magic of the setting and, as I said, produces a gravitational push outwards, so things stick to the "inner wall". Because the gravitational force is magical, I can just handwave it away
For the day, night cycles, I'm going the way of bullshit magicky-science too and have plants use the ambient magic in the air the same way plants use light and some nutrients for photosynthesis
As a result, plants inside this world are all bioluminescent, with varying "on/off" cycles, that create days and nights of varying lengths throughout the world (and allow sentients to cultivate plants to create controlled daylight environment
Most plants also give off heat, so everything isn't cold all the time
Algae in particular give off the most heat when they are stationary, making water in lakes and seas evaporate and recreate a water cycle
Of course the science is so loose that even cholo whores call it "puta" but ... magic, bitch
Another feature is that the different major civilisations (mostly city-states) have built giant towers that stretch all the way up to the Source. They all meet in a kind of agllomerated sky-city where mages and scholars do their business well above the peasantry
This means that nobility, because they are much closer to the gravity source, are much smaller and stockier than peasants, that tend to be slender and tall. This means that midgets are considered paragons of beauty and elegance and that tall slender people are considered hideous and disgraceful.
Still very much a work in progress, if you have any ideas or if you like it don't hesitate to chime in
pic loosely related