Josh Mosqueira has announced that Season and a Ladder-Like-System will be arriving in the first Major Content patch during the Reaper of Souls Release Event. The devs are thinking about adding Seasons-only legendary items, and have hinted at "Tiered Rifts" where players can compete with each other in Adventure mode and fight to the top of leaderboards.
Below is the full information from Blizzard:
Posted by Blizzard: Seasons And Ladder-Like System One of the features you guys have asked about a lot, that we have done a lot of research for, is something called ladders. I'm happy to say that as part of our first big content patch in a couple of months, we will be releasing Seasons, which will include a ladder-like aspect to them. We're still working on them, but the general gist is that you'll level up a new character, we'll have specific ladders for Seasons, and also some goodies that will drop along the way. There's more information of that to come, and we're not making an announcement of when, but that's coming in the first major content patch - not the first patch, but the first major content patch.
Seasons-Exclusive Legendaries There will be more legendary items in the future, both for rewards of Ladders, and also rewards from the Tiered Rifts. For the current ones, we probably are not going to revisit them. They all take time to do and I'd rather we were adding new legendary items, and the time is better spent on new legendary items than going back to refit old ones.
Tiered Rifts And Leaderboards There is another major feature in the patch 2.1, which is going to be what we're internally calling tiered Rifts. The idea is a challenge scenario, where players do increasingly hard Rifts, to see who can get the farthest. You do your bounties, you do a Nephalem Rift. If you clear it, there's a chance to get a Greater Rift Token. If you successfully do that, you'll get a Greater Rift Key (rank 2), then Rank 3, etc.. Much like seasons, we're going to have a leaderboard as well, so you can see who got to the highest tier Rift, and what time were they able to obtain on that.