[1.] Adventure Mode. The possibilities of what you can create in the meta game is endless. (Unfortunately it isn't there yet.)
-With that said, after playing Diablo III: RoS since launch, I will say that the current content is a bit too repetitive and sometimes tedious. For example of tedium, Act 1 bounties being the only thing worth the "specific items" so the other Acts for the most part are neglected.
-Adventure mode outside of rifts and bounties feels like it's missing a lot of content. The only thing in adventure mode right now in the pure essence of the open world, is the infernal machine event. I am very happy they are gonna take another look at this event because we desperately need more content like this outside of rifts/bounties. I really hope the new stuff for the infernal event involves making it useful after you get your ring and amulet. Maybe adding bonus XP/gold/drops.... Remember, you need to farm a key set, so it's not exploitable beyond a 4-player group.
-I feel bounties are tedious in general. Rifting, while repetitive, involves smashing tons of monsters. Bounties on the other hand, I feel a lot of the quests has us running around too much and with run speed so slow, it just feels sluggish. I been suggesting for a long time to increase base movement speed by about 10-20%, would make the game feel a lot more fluent and not sluggish.
[2.] Endless Paragon Levels
-The grind in this game is actually very boring until you find your "builds" item set. For example, playing my crusader was mind-numbing until I got the 1H-Flail and Shield to endlessly throw blessed shield. My barbarian was boring until I got endless Call of the Ancients.... And so on.... I feel the new legendary affix system showed this games true colors in that it needed big change. The combat system is very fun but without a lot of cool things to work for it just isn't there.
-I feel endless paragon is the perfect system to compliment a very good item system. You get the feel that you are always working for something, item or paragon points. The problem is that right now, there feels to be two tiers of gear.
Tier 1: Rares/Basic Legendaries (Boring to do repetitive tasks)
Tier 2: Unique Affix Legendaries / Game Changers (Fun to do repetitive tasks)
I feel there could be more both in between tier 1 and 2 because the time it takes to get your game changing set is too long to justify repetitive gameplay and also a new tier 3, with ultra rare but powerful items, preferably that are different than our normal set of items, possibly new item slots or "rune crafting" our current gear, something to not ruin the cool unique affix legendaries but to add another layer of depth to the item hunt. Here is what I suggest:
[A.] Revamped follower system. Change there item system entirely so there is a new item hunt for both your hero and all three followers. Have all of there items effect your hero in some way, obviously nothing major but also not too weak. Give them 4-6 item slots, each being "follower specific" items. Since these items don't need graphical implementation, it won't be that hard to create. These new items grant our heroes passives, stats, procs, auras, ect... Stuff to compliment our endless slaying. Followers could be a tactical choice in multiplayer games.
Example item stats:
-Chance on hit to reset the cooldown of your followers abilities.
-Your scoundrel searches for extra treasure whenever you open a chest.
-Your templar grants you bonus items when you open a corpses body. (Honor the dead theme)
-Your enchantress grants your attacks a chance to stun treasure goblins.
[B.] Something beyond a full set of "game changing" legendaries.
Once you get your fullset of game changing items that make you a beast, there needs to be an extra layer to work for. Something outside of endless paragon levels and something different from our current item system. A unique system that goes outside the box. Like a rune system but not like Diablo II. Maybe our heroes can get a new item, something to do with nephalem, and we can level up this item to unlock "rune slots"... Runes are ULTRA rare items that grant our heroes passives and random effects. This is just an example of what I'm talking about, could be anything, just something to do after we are fully geared and outside of paragon. Ask yourself, what do we do? What do we work for? We need an ultra rare system past sets/legendaries.... A new item slot or new rune system or something..... Think high runes in D2 (with there rarity) and how much everyone wanted them and then create a unique system in D3 that is it's own system that adds that same level of "I am decked out in gear but I want these high end ultra rare items." And the reason we need this is like I said above, the gap between Tier 1 and Tier 2 is boring. Don't bank on only unique affix legendaries, add another layer past it. Something to really sink our teeth into.
Sorry for the wall of text! I hope someone out there reads it all and sees what I am saying. Thank you!