It depends on your style. I was just introduced to Parallel Keys which is the idea of playing a chord but ignoring the key. For example, Say I want to play a CMaj9, I'd play that for 2 measures then switch to an EbMajor9. Here is where theory would come in. I would improvise over that using the major modes. The only 2 major modes I can use are Ionian and Lydian. (Mixolydian, though major, can't be used because the 5th degree in the key is a dominant 7th which is a flat 7th so that mode would sound bad) If I want to use say the Lydian Mode accent I would find the key to play in in which C is the 4th degree and the key in which Eb is the 4th degree. (G for the first one and Bb for the second) Then, you'd improvise in the with those keys over your chords.

It's a very Jazz Fusion sound.