With Bolt expressing interest early on, it wouldn’t be surprising for Warner Bros. to work in an appearance at some point.
The probability of Bolt playing another speedster in The Flash seems unlikely at this point, regardless of how great he could look in a suit.
Rumored plot details from Famuyiwa’s plan had Flash battling The Rogues, a group of non-speedsters, which would leave little place for Bolt to suit up in any capacity.
Bolt very well could be a bystander on Central City during the film and see Flash race by him on the street.