There is one big limiting factor on what/how much the devs can create: time (as a resource). Something that helps reduce time consumed to develop things for the game is using art, sound, AI, etc. that already exists in the game. Here are some things they could do with the already existing game assets to improve the game:
Rift Race: Play the same instance of a Nephalem Rift at the same time as your opponent(s), but you only see him/her/them as ghost(s). You can do 1v1, 1v2, 1v3, or 2v2. The winner receives a race score (for each combination of number of players) that is on their profile, as well as blood shards or other currencies.Sidenote: The solo vs. group games would be balanced by increasing monster hp for the group accordingly.[/i] Horde Rift: Enter a several leveled dungeon where each level is one room. An increasing number of waves of increasing size (according to dungeon level) occur in each level. At the end of each level (after several waves), an increasingly difficult (according to dungeon level) boss spawns. Play solo or with friends and receive another score for best time on your profile (best group, best solo). Commander Rift:Enter an instance where each player starts in opposite corners. Each player starts with 100 death souls. This currency can only be used in this mini-game, cannot keep after leaving. There are 3 pillars for each player in each corner:
General Pillar: Summon a large minion with a respectively large health pool and damage. Has slow attack speed. Costs 50 death souls.
Lieutenant Pillar: Summon a medium minion with a respectively medium health pool and damage. Has average attack speed. Costs 25 death souls.
Private Pillar: Summon a small minion with a respectively small health pool and damage. Has fast attack speed. Costs 10 death souls.
The players do not fight each other, they fight the minions summoned by (and at) the pillars. The other players are ghosts in this mode, only visible (not interactable).
Killing minions grants death souls (5 for a private, 12 for a lieutenant, 25 for a general).
The player and minions can attack opposing pillars, which have heath pools. Once dead, the pillar no longer summons anything. Once all 3 pillars are dead, that player loses and is eliminated. If eliminated, you can ether spectate as a ghost or leave and collect your currency reward (blood shards, whatever). Placing higher in terms of 1st, 2nd, 3rd, or 4th (depending on the number of players competing) determines your currency reward.
You will get a score on your profile for this, too.
Picture Time:
All of the above game-modes/mini-games are special rift keystones that need to be found or rewarded in the world. Obviously opening a portal with your keystone opens a portal for the whole game (all of the players). When one is joined, other players will be given a request if they want to join to, regardless of where they are, much like act boss encounters.
Rift Race: Play the same instance of a Nephalem Rift at the same time as your opponent(s), but you only see him/her/them as ghost(s). You can do 1v1, 1v2, 1v3, or 2v2. The winner receives a race score (for each combination of number of players) that is on their profile, as well as blood shards or other currencies.Sidenote: The solo vs. group games would be balanced by increasing monster hp for the group accordingly.[/i]
Horde Rift: Enter a several leveled dungeon where each level is one room. An increasing number of waves of increasing size (according to dungeon level) occur in each level. At the end of each level (after several waves), an increasingly difficult (according to dungeon level) boss spawns. Play solo or with friends and receive another score for best time on your profile (best group, best solo).
Commander Rift: Enter an instance where each player starts in opposite corners. Each player starts with 100 death souls. This currency can only be used in this mini-game, cannot keep after leaving. There are 3 pillars for each player in each corner:
All of the above game-modes/mini-games are special rift keystones that need to be found or rewarded in the world. Obviously opening a portal with your keystone opens a portal for the whole game (all of the players). When one is joined, other players will be given a request if they want to join to, regardless of where they are, much like act boss encounters.
Edits: Formatting