Some substitution ideas (playing a chord instead the normal one) :
Neutral chords (modal)
You can replace all the chords that you play by a neutral one (chord with 2 notes, 1 and 5, call "5" or modal).
Each chord has his relative minor. It's the minor chord built from the 6th note of the major scale.
For example for D it's Bm, for G it's Em, for A it's F#m etc...
You can replace a minor chord by a minor 7th.
Example: Bm (B + D + F#) = Bm7 (B + D + F# + A). So you can replace a major chord by his relative minor 7th. Example: Em7 instead of G.
7th major chords can replace minor chords and major chords.
Example: GM7 instead of Bm or DM7 instead of A. To understand this substitution, you need to see the common notes between the chords. For example Bm = B + D + F# and GM7th = G + B + D + F#, there are three notes in common. For major - major 7th substitution you go further, for example A = A + C# + E and DM7 = D + F# + A + C#, there are just two notes in common.
major 7th (7th minor)
Major 7th chords can replace chords of the 5th degree of the major scale (Ionian).
For example when you play D - D - G - A, you can add the 7th minor to the A chords, so it will make A7 = A + C# + E + G.
Sus4 chords (suspended four) can replace chords of the 4th degrees of the scale. In that chord we replace the third by the fourth.
For example, you can play Dsus4 instead of a G. Let us look at the common notes: Dsus4 = D + G + A and G = G + B + D, there are two notes in common.
Major or neutral chords
We can substitute a minor chord by a major or modal one.
For example, in the minor scale we can play G instead of Bm. Bm = B + D + F# and G = G + B + D, there are two notes in common.
In the Dorian scale, instead of a Am we can play a D major or modal. Am = A + C + E and D = D + A + F#. There are two notes in common. In the A Dorian scale, you have the F# note and we find that note in the D major chord. Sometimes we play them modal, so as not to shock too much !
A 6th chord can replace the relative minor of the major chord.
For example, instead of a D we can play a Bm but also a D6. D major = D + F# + A, Bm = B + D + F# and D6 = D + F# + A + B.