Just like atomic said it's nice to add 7's chords... but when doing it i think you better use the mixolydian mode because in this mode the 7 note of the major scale is pulled down by 1/2 step, and in X7 chords
the note that added is also 1/2 lower than the 7 note in the scale...
i hope you understood me
P.S in minor it's nice to play with 5's chords