They added set dungeons and new mob types. They're also reworking sets every patches so you can play classes differently, since sets are most of what defines classes nowadays. With LoN, they even added a way for you to play how you want without being tied down to sets(although it requires ancients in every slot more or less).
The new grind isn't really a grind for casuals, it's a new grind for bored people who play a lot and end up not having any use for the gem upgrade shit at the end of GRs cause they have only 1% chance to upgrade their current gems anyway. So instead they do 3x upgrade of a garbage gem, and once the gem is high enough, they use it to buff a piece of gear. Then they do it again. By doing this, bit by bit, they can do higher GRs, which let them upgrade their gems again until they cap out at 1% again, then they level garbage gems and so on. You can tell the system isn't designed for casuals in mind since it only works on ancients, so you need full ancients to benefit fully from it.
As for adding bosses, I disagree that it'd be an interesting addition. They could add new stuff, but bosses are boring as fuck. They tend to be super easy, especially if you spec for boss fighting. You could say "oh well then make them hard", but hard for who? Most of the D3 players can't even do T10, they play like 2hours a week and are still farming to get their actual build full items, not even talking about optimizing stat rolls or getting ancients. Then on the other end of the spectrum people with a lot of time have super high paragon, full ancients with fully upgraded new stat cube thing. The difference between the 2 is absolutely ridiculous. Mobs have several hundred times more health in the GRs the later group is doing compared to the casual. If you balance it for one group or the other, then it's gonna be shit. You can tie it to Torment level you say, but that's the same as balancing it for casuals, since T10 is shit. They could increase torment levels again to compensate, but it gets confusing for casuals when there's like 16 torment tiers, and it's unlikely you'd get torment levels as high as the higher GRs, just like T10 was pretty far too.
At this point what they probably need is a reset, wow expansion style. Which is why we need an expansion. That would also add new bosses, and they can rework old fights to be more interesting and shit. Uber Diablo was a much more interesting fight than any of the RoS bosses fights. Sure it was just reused bosses, but fighting several bosses is a lot more interesting than fighting a single one because it's harder to keep track of the patterns.