I could see Oden and Mihawk seriously eyeing each other and dueling the entire time. Oden would be 59 in the current story line, a bit too old be WSS but still a very challenging contender. A character as notorious as Oden just forces Garp or Mihawk into the front lines. Sengoku might even assign a specific Admiral to thwart Oden's advance the entire time.
As other said, Ace was already saved thanks to Luffy breaching the platform and he died protecting Luffy from a battle that shouldn't have taken place. This doesn't change, Oden is not Ace's bodyguard.
What's more interesting is how Oden would handle the death of Thatch or Blackbeard's arrival in Marineford. This is a man Whitebeard definitely can't hold back with a MARRRCOOOOOO. Oden goes after Blackbeard in both scenarios, especially if he was the former 2nd division commander where BB served. He should comfortably slay Yami Teach before he can acquire the Gura Gura, but I wonder if the Impel Down prisoners can gang up on him and let Blackbeard suffer before dying Whitebeard.