I think he should be a hired criminal by Harry to steal something from a nuclear plant or sumthing simular. maby harry needs it for improving his dads goblin chemical. spiderman should try and stop him but mess up badly and set off some kind of strange melt down. running from the melt down Flint could be on the beach heading for the get away boat when he gets fused with the sand.
or maby he was trying to steal the symbiot from a lab or something and a "melt down" happens. that way u get the Sandman and Symbiot origin in one shot.
as for the symbiot, what if it was something descoverd fosselized (i know i suck at spelling) from either a meteor crash or simply somethign that has been on earth for a while but dormant. then the melt down that creats Sandman could jump start the dormant symbiot and bring it back to life. later in the movie you would find out it is of alien origin.
or maby he was trying to steal the symbiot from a lab or something and a "melt down" happens. that way u get the Sandman and Symbiot origin in one shot.
as for the symbiot, what if it was something descoverd fosselized (i know i suck at spelling) from either a meteor crash or simply somethign that has been on earth for a while but dormant. then the melt down that creats Sandman could jump start the dormant symbiot and bring it back to life. later in the movie you would find out it is of alien origin.