I agree. If it is Diablo 4, then it is most likely employing a relatively new engine, possibly the same one that is being used in Overwatch. This would obviously allow the devs greater flexibility for just about everything.
One such area, of course, would be for cosmetic items. Diablo has never placed an emphasis on character customization and aesthetics, but I think that could certainly change in a future Diablo title. A new engine coupled with more detailed art assets for weapons, gear sets, armor, etc, could allow the team to introduce cosmetic variations for just about every item in the game. And certain cosmetic drops could have rarities, just like the actual items.
For example, different aesthetic variations of class armor sets could drop from certain treasure goblins, or could perhaps be found in chests randomly generated on maps. Add in portraits, followers/companions, and wings, and suddenly, you have quite a bit that can be monetized to allow for a continuous stream of revenue well after the point of sale.