If not Rock, then who faces Roman at mania? A Daniel Bryan angle virtually writes itself. Roman complaining how the 2015 Rumble was supposed to be his breakout night only to be ruined by the crowd wanting Bryan would be an amazing promo. Maybe Big E? Although he probably isn't ready for that spot just yet, not like that's stopped them before from a too soon mania push (Roman WM 31).
Big E would be fine. They can build him up and Roman can beat him and look badass. They shouldn’t even be thinking about taking the belt off him for a very long time anyway, so the goal is to find credible challenges who can lose and still stay over.