We're actually looking to expand upon Nephalem Rifts in our upcoming 2.1.0 patch, creating what we're internally calling "Tiered Rifts." In a way, Tiered Rifts are similar to an endless dungeon (or "topless tower," as Wyatt likes to call it) in that as you complete different tiers of Rifts, you can unlock more tiers. Another goal with Tiered Rifts is to provide a way for players to test and challenge themselves, and also compare their progression against the progress of other players.
Wyatt gave a sneak peek during our Live Q&A on March 24:
Wyatt Cheng: 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. The way it works is: You do your Bounties and get the tokens, then 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); if you clear that one, you get 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.
Tiered Rifts are still in development, so there aren't many additional details to share right now. We hope to provide more information in the coming weeks, though--so stay tuned!