Given the nature of legendary drops and min-maxing, one thing that would be really nice is a way to easily see the range of possible stats on an affix. Waiting for Diablo 4 to come out I was playing V Rising (for an in game example) and when you got a drop that say “Increases chance of blank by 6%” you could just hover the item and hold shift and it would show you the possible stat rolls, ex: “4%-10%”, so you instantly knew, relatively speaking, how good of a roll you got on the item stats.
I could see arguments go either way on something like that in Diablo, but players are just gonna create wikis for it anyways so something in game would be really nice. Again, specifically for the legendary affixes, not necessarily all stats.
Another nice feature would be an improved Codex that tracks all affixes once you’ve found them. Currently it shows the powers you can find from dungeons, but a growing list as you get more drops would be ideal. Also a search function; there’s a lot of stuff.
Loving the game so far 
Edit: Apparently it is in the game, hidden as a setting “Advanced Tooltip Information” under Gameplay. Good to know but weird it’s hidden when it’s pretty important information. Thanks everyone!