SQL & PL/SQL :: Function Returning Table Of Dynamic Structure

I have a requirement to be coded like this:

A function to return pl/sql table(cant use ref cursor) whose columns varies every time it runs i.e,

means
type pl_tab_type is object(col1 varchar2(1000), col2 varchar2(1000))
type pl_tab is table of pl_tab_type

func f return pl_tab
as
...
end;

note : pl_tab_type will vary for each run of function f

i.e.,for example, pl_tab_type can be changed to as follows:

type pl_tab_type is object(col1 varchar2(1000), col2 varchar2(1000),col3 varchar2(1000))

how to return pl/sql table of dynamic type from func,