simple suggestion.
Whilst in the slab mode of building push a key like shift and drag up to make a vertical slab.
You could also have a button for diagonal lines like ctrl.
you would then be able to hold ctrl and shift in a certain order to make either a diagonal wall or a slant for a roof.
Sounds simple in theory, but how can the game tell? You dragging up in the screen cound very well mean making a horizontal floor in that direction, i bet the team can make it via a button like the rotation keys, but i think this’ll come after the UI changes
(Also, a little low on priority right?)
Just have in the building mode to select slab height. I would kill for something like this.
I hope this feature would be assigned to the shift key myself, because that’s what RCT2, RCT3, Parkitect, and Planet Coaster all use to raise/lower objects instead of moving them horizontally. I hope I’m not the only one used to that 
You could also have a button for diagonal lines like ctrl.
you would then be able to hold ctrl and shift in a certain order to make either a diagonal wall or a slant for a roof.