How difficult would it be to implement on-the-same-track transition? I'm talking about the VSE. Right now you have to waste 3 (3!) tracks in order to make a transition between two clips; the first two of the tracks you use for video strips and the third one is for a transition strip itself.

As far as I know, there is no addon that let you make transitions on the same track. If there is any, please let me know. I don't know is that even doable with python or you should code it in core components and compile afterwards.
If you are transitioning from one clip to another, you only need one track. Position the tracks in the order you want. Select the first clip, then shift select the second (both tracks selected). Now add a cross effect. Right click on the end of the first clip or the beginning of the second clip. Now hit "G" and move the selection to the desired length for the transition.