With its unique ability to transmute items into new ones, the Horadric Cube was one of the most useful and iconic items in Diablo II. And while Diablo III had no item like this so far, that will change in an upcoming content patch.
According to the official preview of patch 2.3.0 notes, there's a new Cube coming with similar abilities to its Diablo II counterpart. It's called "Kanai's Cube":
Those who played Diablo II might remember the Horadric Cube, a unique device which allowed you to combine items. Useful as it was, it was easily surpassed in power by the item from which it originated, Kanai's Cube. In Patch 2.3.0, players will be able to discover this powerful ancient relic and utilise its incredible potential, including the ability to break down Legendary items and equip their special effects as passive skills (which are separate from your other passive skills), convert crafting materials from one type to another, and so much more.
Good news! It might mean we no longer have to wear that bloody Ring of Royal Grandeur on every character. This ring has a very powerful passive ability (it reduces the number of items needed for set bonuses by 1 -- to a minimum of 2) and it's basically a must for everyone in higher difficulty levels with no real alternative. With Kanai's Cube, we could just break it down and use its special effect without wearing the ring.
There'll be a few more minor changes in patch 2.3.0, and you can read the whole preview here. One of the most interesting one besides the Cube is the introduction of four new difficulty levels: Torment VII, VIII, IX and X. Sounds crazy!