Hi and welcome.
To edit WMOs, you need Blender and Blender WMO plugin. I haven't managed to cover this topic completely yet (mostly because plugin is still under development and its worthless to make tutorials for unfinished software), but here's at least something.:
You can use this plugin to open WMO in Blender and edit it as you desire. If anything doesn't work for you, I'd recommend contacting directly Skarn, current plugin's developer.
A new continent creation is described here:
Loading screens are refferenced in map.dbc. For maps, zone music is being set by applying AreaID in Noggit, and areas are stored in AreaTable.dbc. Zone music is attached to those areas. For WMOs, you need to set DBC Group ID (WMO area ID) to a value from WMOAreaTable.dbc you want to use. You can create a new WMO area, or use already existing one. About "weather"... there are no zone weathers, but there are things called skyboxes, which are actually combinations of cloud, sky, fog, lighting and water settings (maybe something more). Skyboxes are "spawned" into the world in Light.dbc, you basically add spheres of sky settings around the world as you desire.
I have no experience with Legion modding. There are some guys attempting it, sure, so at least something is definitely possible. But most of people either stick to WotLK, or to WoD. Legion is still new, still unfinished, and definitely not an easy start for a beginner.