So here's a thought. Play CE BD (Cmaj7/9). Now play EGD. That will be eminor7. What do you notice? That's right, these chords share some notes. You can put in an eminor in place of a C major.
I have a progression where I go VII, I / II, I / IV, III / IV, II. Originally I had written his as IV, I, IV, I, IV, I, IV, II.
The VII chord is one I'm using for the first time: it is a minor chord with a flatted fifth to boot. That shares some notes with the II, which in turn shares some notes with IV.
I haven't used the sixth degree (relative minor) very much yet, except as passing chords. But the beauty of this is that if you know one or two keys, and seven chords in those keys, then you will find those chords like old friends in new keys you try to learn.