Why not introduce a benchmark mode, which allows you to measure the effectiveness of different builds by replicating different scenarios? Imagine a Realm of Trial kind of small map, where, depending upon your choice, you're put up against either a group of enemies or a single boss. After defeating the enemies, you're shown a statistics screen (think of Greater Rift completion screen) with your average damage output per second, your highest critical damage, damage taken, amount of energy spent, etc-etc. All this not only to test out the combination of different gear and their synergies, but also different skills.
Right now, some of that can be achieved with the pen-and-paper approach: you choose a map that offers the nearest scenario of monster density you want to test and then start killing stuff, during which you try to memorize/write down those numbers flying out of your character's head and then try to reach somekind of conclusion. Sure, this works for trivial cases, like which weapon can produce higher average critical damage numbers, but it would be great to benchmark, for example, which gear setup offers you fastest area clears or most damage with the least amount of energy spent.
I think this would encourage casual players to experiment more with the gear they have in their stash (especially now that we have Kanai's Cube), instead of just grinding for an ancient version of the items they already have.
Another possible use case for this can be competition besides Greater Rifts. People can compete with each other in the following areas (just a random list of things off the top of my head):
Fastest single enemy clear
Fastest enemy group clear
Biggest single hit damage
Fastest attack speed
Fastest energy regeneration (say, from zero to full)