Make all the gear the same that's on the same "tier" of difficulty, then make the gear stronger when it is being used in its activity.
So you get a chest from a normal raid that is 100 ilvl and you get a chest from M+ (i'm not guessing a number :P ) that's 100 ilvl. Out in the world the two are equal if the M+ player goes into M+ his gear is 110 if the raider goes in M+ his gear is 100 still, same story for raids when flipped.
Now neither person is weaker in their content nor do they have any inclination to do content outside their interest although if they do their gear isn't useless it's just not as good as someone who always does that content. They can still overpower the other content by doing higher difficulty tiers, a heroic raider will have better base gear than a normal raider (like now) so they'll still do better than them AND better than someone who only did a M+2 but not better than someone who did a M+(not falling into the trap :P ) which is on the same "tier" as heroic raiding.