Hi Blizzard here is some feedback for Diablo 3 RoS, 
Nephalem Rift doesn't give a strong incentive to play right now, doing bounties is by far the best experience. Rift keystones have a lot of potential though. It is to easy to get rift keystones, perhaps combining 10 or 20 into a legendary rift gate with great loot and difficulty would give that extra incentive to play it.
I think the end-game bosses should be more part of the bounties objectives, right now you never have to kill bosses such as Butcher for bounties, it would be nice if that would be added. There are a few timed quests which I like, such as finding the treasure room. Perhaps this can also be implemenetd for killing bosses and such, perhaps optional but if done rewards would be bigger.
There is still a lack of end-game. I think this is a major issue. Giving the ability to combine rift keystones into legendary ones with great rewards will give an extra reason to play but ultimately endgame will be the game's legacy.
End-game could be provided through pvp, that is the easiest solution but due to how long it takes for it to be implemented I expect this isn't very easy to get right. A pvevp system where players compete against eachother through killing monsters might be a  nice solution. For example you have teams enter a room each side and they can  activate certain waves, harder or easier ones, more damage or more tanky ones and by slaying the monsters they increase the difficulty / add attributes to the waves of the opponent. For example, as you know your opponents' setup, if they have a very high dps intensive setup you activate reflect damage on all mobs by triggering a wave at your place. By doing it through different waves you can add modifiers to monsters on the opponent team making each wave harder. The one team where everyone dies first loses. Add to that a reward system or ranking ladder and you have a strong endgame similar to Hero Line Wars, a Warcraft 2 map mod.
Another idea is an endless wave gamemode with wave checkpoints so you can start off where it gets interesting with more rewards the further you get. Spawning strong bosses at certain intervals would be a nice bonus.
I played Path of Exile for a while and I think many ideas from that game could improve Diablo 3: RoS a lot. A ladder system they took from Diablo 2 for example, also how hardcore characters are moved to softcore when they die. I like the two hourly races as well giving people points and allowing people to get unique rewards.
I also don't like so much that there is no cap in levelling, this takes away an objective as hard it might be, even if the cap is 1000 paragon levels and takes 10.000hours having a cap makes people psychologically aim for it.
Finally I want to emphasise how important end-game is. With the right mechanics that don't require too much development you can turn a 10-50h experience into a 500-1000h one.
I would love to offer more in depth help, if there is anything I can do let me know. I'm an advanced theory crafter that broke a lot of records in the diablo universe and wouldn't mind working certain ideas out. Having graduated as an IT programmer I also understand the challenges to code certain features and would keep things as simple as possible but increase the fun/lifespan a lot.
Cheers,
Chiren/Athene