Likely, what you are thinking about is having single method in the factory that contains a switch statement and takes a parameter (either a string or integer, maybe expressed as an enum) and returns a Slide object.
Other methods would then call the factory with the correct enum/int/string value to get a Slide object, and then either use the returned slide or pass the slide to some other object.
But here's a thought... Those other methods know exactly what slide sub-type they need, after all they have to understand which enum/int/string value maps to the correct slide sub-type. So instead of them calling this factory method to get the slide, have them create the correct slide sub-type directly.
With the above, you don't need the factory and you don't need to modify any function to add a new slide-subtype.
This is the OO way.
That said, if the int/string/enum value is coming from some configuration file, or other external source, then there is nothing in your code that knows what slide sub-type to use. In this case, you would need the factory function and you would need to modify it whenever adding a new slide sub-type. Some languages have a way of adding a new sub-type and updating the factory without having to modify any of its methods. I'm not sure if dart/flutter is one of them.