I think it would be nice to have a tab where blocks of code could be copied temporarily, while trying out other code. The tab should be saved with the project, but not compiled.
I think this would need to be done carefully to avoid confusion. We are accustomed to having all our code compiled. I do actually use a similar "Scratches" functionality in the PyCharm IDE a lot, but that is an IDE targeted to advanced users with many advanced functionalities that would not be appropriate in the Arduino IDE.
The Arduino Web Editor has support for adding documentation files to the sketch. These could certainly be used as a "scratch pad". The IDE does not currently have support for this though. You could simply add a tab to the sketch and disable the contents with a preprocessor conditional that will never evaluate to true
// Scratch code here
#endif // false
Then it would be wonderful with a few "user" buttons where board and port could be selected, instead of having to go through the menus each time. Right now I work with communication between a micro and a NodeMCU, and have several IDEs open simultaneously.
In Arduino IDE 2.x, which is now in beta development phase, each window can have a different board and port selection.
In the classic Arduino IDE, you can achieve the same thing by starting a separate instance of the Arduino IDE for each window. This is done by just starting the Arduino IDE directly multiple times. It's only when you open multiple windows of the same instance via File > New, File > Open, File > Examples, or File > Sketchbook that the board and port selection of one window determines the selection in the other windows.