I should be allowed to edit read-only files within Notepad++, then be prompted with a Save-As prompt stating "This file is read-only; you must save it as a new file to save changes" if I try to save it.
This is particularly important to me because I often use Notepad++ to view generated temporary files which unexpectedly have the read-only attribute turned on in the operating system. I often wish to do some editing on this file (perhaps to clean up some text before copying and pasting it elsewhere, or perhaps I wish to save-as AFTER I've made changes), but when every key press fails to make any changes, I forget about the "feature" of Notepad++ completely locking the file and I waste a lot of time trying to figure out why my keyboard suddenly isn't responding.
Once I realize that my keyboard is not broken, I end up selecting all text, copying it, and pasting it into a new tab (and selecting the correct language for that tab), which is far more effort than I should have to put into fiddling with a read-only temporary file.
Or at the very least, Notepad should display an annoying alert indicating that it's not going to let me modify the loaded text every time I try to add or remove text.