The Steelers' national brand recognition power is strong, so it may not be limited to typical local-based corporations. Pepsi already has a long-standing advertising presence with Heinz Field and the University of Pittsburgh, so I would suspect they may be a player. If a powerhouse like Google wanted into the game, the Steelers could make sense, but such sponsorship isn't really the typical marketing approach for such companies.
Among the local companies, UPMC is the obvious one, along Alcoa. Dick's and AEO could have been prime candidate 10+ years ago, but the retail world's seeming demise and future uncertainty probably takes them out serious consideration.