Resident Evil 6 will probably feature Ada Wong as a playable character. During the QA portion of the RE6 Comic-Con 2012 panel, someone asked if Ada Wong would be playable in the game. The Exec Producer via a translator answered "Would that make you happy?" "I can't answer that now but I might be able to make you happy soon," teasing an upcoming announcement that would confirm Ada.
We already know Ada is in the game, but as of now she's only making an appearance as an NPC. Earlier this month there were reports indicating there may be a 4th campaign. Files uncovered in the Resident Evil 6 demo suggest this campaign will feature Ada Wong.