In Basic Chord Substitution, we mentioned parallel substitution, also known as modal interchange. An example of modal interchange is using the bVIIMaj7 chord in a major/ionian key, a chord which could be considered borrowed from the dorian (b3, b7) or mixolydian (b7) scales, for example.
bVIIMaj7 in use (BbMaj7-G-C)
Take a look at this chart of the modes of C harmonized in 7ths:
Basically, chords sharing a column are ripe for substitution via the modal interchange method. There are of course many more scales than this, and after a while quite frankly it starts to mean that just about anything can become just about anything. That's cool. But it's nice to have reference points, islands plotted along charts of the journey, if you will.
Here are some popular modal interchanges not covered in the chart:
IMaj7 to I-6 or I-Maj7 (IminMaj7)
IVMaj7 to IV-6
V7 to V7(b9)
Again, there's really no end to this kind of discussion, and it comes down to experimentation, to what feels/sounds right to you.