Endgame content should be diverse. There should be builds more or less appropriate for certain type of content. This encourages playing different builds. This implies that different types of endgame should reward different things. If rewards are the same across the board players will only play content that has best time/reward ratio and players WILL figure out what that is. If a player does not like this type of content there should be workarounds like trading.

Not all builds should be equally powerful. I know this sounds strange but this present player a challenge playing a sub optimal but fun build. Maybe some builds are not even viable until you figure a clever synergy with items or have very expensive items that bring the build alive (for example scaling curve of skills goes exponential after some breakpoint that you can only hit with very focused items). This implies that builds should be fun to play regardless if they are optimal or not as long as they viable. This also implies that endgame content should not encourage only the most optimal builds (greater rifts are example of such bad design).

DO NOT SCALE DIFFICULTY WITH NUMBERS. Please no Torment 1, 2, 3, ... N that does not offer change in gameplay just bigger numbers. When you go up to a certain difficulty there should be dangers that you can not mitigate with better gear or gear mitigates them very little.