WWE has had plans for Roman Reigns to win the 2015 Royal Rumble and go on to defeat Brock Lesnar for the WWE World Heavyweight Title at WrestleMania 31 but there is a lot of skepticism on whether or not Reigns is ready for a push that big.
While Reigns may need some promo work and isn't that good on his own in lengthy singles matches, WWE is in need of a new top babyface star and that spot is his. There were plans for Randy Orton to to continue turning but Orton isn't a new star. Daniel Bryan is not WWE's type for the main event role, no matter how fans react to him, and there are still uncertainties regarding Bryan's future due to neck and elbow injuries.
That leaves Reigns and Dean Ambrose as the new top babyfaces. Ambrose is a better in-ring worker and delivers better promos but he fits less into WWE's mold of what they want their top babyface star to be. While Ambrose and Reigns both do strong merchandise numbers, with Ambrose's new hoodie being #1 during Thanksgiving, neither is doing numbers strong enough to be a top guy.
However, WWE will likely stick with the plans for Reigns' WrestleMania push as he's still considered the new top babyface.