TL;DR
Allow the player to switch between freehand lasso and rectangular lasso for the selection tools.What ?
I suggest adding a way (probably a keybind) to switch between the rectangular lasso (that currently exists) and a freehand lasso, for all the existing "selection tools", e.g. deconstruction/upgrade planners, blueprints, and modded selection tools.The shape is made closed by connecting the start and endpoints, for instance if the player drew the red line below, the blue line would be used to complete the shape. This would also make it so that drawing just a single line would deconstruct just entities in that line, which is often useful with the rectangular selection tool (for belts) and would now be usable for diagonals as well (rails).
The selection should exist in the world space, so the player can walk around while making it in case it doesn't fit on one screen (though this would make zooming in do unexpected things if you weren't in the map view, maybe disallow zooming in this case).
It might also be a good idea to shade this region so that it's more clear what it's going to affect.
Why ?
- Selecting things near diagonal rails (or diagonal rails themselves)
- Copying part of a build that isn't rectangular