modifier keys? You refer to being able to do, for example, CTRL + left mouse click. Or simply right click or central for another order? Because I also miss this. And also the "released" of the keyboard keys.
I suppose that selecting several notes to lengthen or shorten them would be a typical operation to configure chords.
If you make a mesh of frames (buttons) with the ability to insert notes, or delete notes with the same click, it is very quick to change the length of several notes. If there are 3, it would only be 3 clicks. In the end, many operations can be solved with several clicks, and that would avoid having to select an area, and then apply a specific operation on it (lengthen or shorten the notes). I'm just discussing things. I suppose that using blocks (rectangles) to modify the duration of the note is something sacred for many pianorolls (because all DAWs use that). But it does not have to be like that always. Cubes can be used at any time. At all times I am talking about something functional, understandable to the eye (one cube, represent the trigger note, and another cube the note-off, would be the most faithful representation to Renoise).
Regarding the overlapping notes. Do other DAWs allow you to play the same note 12 times on the same track of the same instrument? Maybe I'm kind of lost here. I know that other DAW's have problems with layer overlap too. But I do not know if they are so similar to the amount of information that Renoise can show with its 12 note columns. In a pianoroll a note corresponds to a square. You have 120 boxes for 120 notes. With Renoise you have that multiplied by 12. From my point of view, with Renoise there can be many cases where information is lost (vision of the notes) by that "superposition of layers". Other DAW's will also have this problem of overlapping notes, but is it possible that they do not have so many combinations? It is clear that if you use the pianoroll, it is likely that you will not superimpose notes, and if you do, you will see that something strange or different is happening.
Regarding selection, drag and other operations, it is something more complex. I have not used it. I have invented the possibility of editing by blocks (groups of lines), for a note column (selected ones) or for all note columns. It is more limited than a selectable area, but for most operations, it works very well, even with cutting, copying and pasting. At all times I have tried to make it as simple and functional as possible, without being influenced by the rest of the horizontal pianorolls. I think it's not about "copying" what other pianorolls offer. It is possible to be something original with what we have.
When you have the roll in front, and you realize how easy it is to change the tone of a note, with just a click (nothing of drag), you realize the power of the mouse. Up to now it is the most interesting tool that I have built, because it has allowed me to think how to do certain operations in another way, such as modifying notes without dragging, or a reasonable selection method that is very simple to use.