So I think it is probably safe to say that another Diablo expansion is inevitable. And while Greater Rifts are a fine little idea. I still don't like the in and out gameplay of them. I don't like being taken out of the action until I WANT to be taken out of the action.
So I propose an endless dungeon mode.
In this mode you enter a randomly generated map that continues on for as long as you want. Think a rift which doesn't have a progress bar. You move through maps continuing through floors, allowing for constant battling and loot until you want to stop. Difficulty in these dungeons are simply static for whatever game difficulty level you have the game set to.
Then there is also an idea I have for "Challenge of the Endless" to make this dungeon concept interesting and even competitive.
So in "Challenge of the Endless" mode:
Every five or ten floors a boss monster spawns and must be defeated before players can access the next ten floors.
Every 10th floor boss spawns a chest in which you can access your storage, allowing for inventory management. (This happens in standard Endless mode as well to prevent players from being forced back to town because, again, it serves to break up the action.)
Every boss killed increases the difficulty of the next ten (or maybe five). Since the dungeon is endless, eventually you will get to an unbeatable point. Leaderboards can be set up to see how many floors one can get. I know this sounds like a copy pasta of Grifts, where the difference lies is that Grifts are sprints of a single small set of maps to race to the end as fast as possible. The Endless Dungeon is a marathon.
Players are given a total of five deaths per Endless, keeping players from ticking away through high levels death after death. Once all five deaths occur, players return to town and must start an Endless Dungeon all over again.
After level 50, the player will gain a keystone that upgrades every floor. This allows players to resume their progress should they have their D3 play time cut short. The keystone is usable only once. However if a player gains uses a keystone to resume a Dungeon they will gain a new keystone upon progressing another 10 levels. For example, if you come back to a level 53 dungeon, you would gain another keystone at level 63. All keystones are destroyed if a player uses up their allotted deaths.
Every five levels beyond level 105, a random debuff is applied to the player. These can range from things that reduce spender damage by 25% to a random non-weapon piece of gear "breaks" locking out stats and legendary item bonus (if applicable). This will promote repeat play to see if players "get lucky" with their debuffs as if you get only minor ones or ones that don't apply to your build, letting that push players another level or two into the dungeon.
The dungeon mob difficulty caps at level 100, making the debuff the factor that ultimately kills the player. This difficulty would be equivalent to TormentXV.
So in essence you want to make the new feature almost exactly like Grifts, except instead of a timer to stop you, it's total deaths. Like the poster above, useless in HC mode, and it's so much like Grifts that even for SC mode I wouldn't personally vote for it.
Perhaps with a tweeked rule set and reward potential.
How about instead of dying in Endless mode, you get send back several levels and all the mobs reset? IF you get kicked back three times, the dungeon closes and you must start all over. This way HC don't die and there still is a cost to dying.
While mobs and bosses on each floor drop normal loot modified by the difficulty of a dungeon level (torment equivalent loot modifers), once you finish your dungeon you get a loot bag with items based on the highest reached level that session.
Special Items include:
Ancient emblem. (When combined with a legendary in the Kanai's cube, that legendary gets rerolled as an Ancient item)
Medallion of the Endless: Legendary Amulet with the property "Grants wearer an addition 100% while in an endless dungeon"
Merchant of the Endless: This items summons a vendor that sells every set item in the game. But the player can only buy one item per summon. Any item purchased rolls itself randomly as if the item just dropped.
Bottomless power potion: When used this potion doubles the legendary effects equipped to the player for twelve seconds.
So its kind of like a Grift for people that dont care about leaderboards like myself. Grift 60 is uselss to me if I try hard and beat it and end up with gear for another class and some more veiled crystals. I am all for a new mode with more loot potential but people arent going to go for it if their is not enough of a difference from a regular Grift or rift. But yea I wouldnt shoot that down, its a hell of a lot more useful than the pvp arena. I personally would like to see an arena that helps you theorycraft, stats, test out skills, see what is actually doing damage, get some solid numbers and testing going on.