Probably a reality manipulation device, a SourceTM engine/addition to the TorqueTM Mission Editor that allows you to directly affect sun and lighting in real time, and a modelling/map making program made specifically for Blockland.
Well.
We can dream.
Not to be rude and anything, but Source engine is not for games that require large open environments, like Blockland. Whereas Blockland terrains are basically infinite, Source can't achieve that with brushes. It's more suited for closed areas like corridors, hallways, city streets, maybe a smaller railyard at most.
Regarding what will be in v19, I'd say it will be bugfixes mostly. Though
Well.
We can dream.