Most are gotten from completing Road to Wrestlemania unless it states Universe Mode.

Unlockable                       How to Unlock
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Arn Anderson's Civilian Attire - Hero Story Scene 5-1
Dashing Cody Rhodes            - Villain Story Scene 4-1
Drew McIntyre Suit Attire      - Villain Story Scene 13-2
Edge Entrance Attire           - Villain Story 15-2
HHH Street Attire              - Outsider Story Cutscene
John Cena Entrance Attire      - Villain Story 1-4
John Cena Purple Attire &      - Play two different Wrestlemania Matches featuring 
                                 Cena in John Cena with T-Shirt Universe
Kevin Nash Suit Attire         - Hero Story 14-1
Mr.McMahon Suit Attire         - Hero Story Cutscene
Randy Orton without Beard      - Go into a Wretlemania Match with Randy in Universe
Sheamus Suit Attire            - Villain Story 18-1
Sheamus T-Shirt Attire         - Villain Story 5-4
Undertaker Hooded Attire       - Go into a Wretlemania Match with Undertaker in Universe.
Wade Barrett Suit Attire       - Villain Story 19-2
William Regal Suit Attire      - Villain Story 8-5