I tend to play ideas off the whole tone scale for a chord like D+(7?), but you can also experiment with different sounds to see what sounds good to you. There are a lot of interesting colors you can get through exploring altered chords. Same applies to Gmaj7. Playing off the IV7, ii, the tritone sub F#7, the #11 of G major, and/or the whole tone scale itself works just fine. I'll have to sit at the piano and voice through those two chords to more clearly hear the sound. In any case, run the chord tones above the 7th so you strengthen your ears to hear the upper extensions of a chord. That in and of itself gives you more to work with conceptually.