Already Played Songs + Manual Automix

My first idea is about to mark with a “tick” or something else the already played songs and force the programm not to play them again on its own, only if the user wants to. That should be easy.
Here comes the hard part and my second idea. You can create manual automix. Let’s say that we have a track with 3:00 min length. 2:40 is the real deal and the an other 0:20 is silence. If there was a pointer that could be moved to 2:40 in the waveform that would tell automix to start from there it would be a major update for your software. Also for evey track you manually automix (set a pointer) you can automatically save that and the next time you’ll use that track, the pointer will be there and automix it by itself !!

An minimum begin of mix at the end of song “marker”
A absolute middle of fade at the end and begining of songs, that have priority,
A absolute end of mix “marker” at the begining of song,

That’s the real deal about that, that i would like.