nevermind about framedoubling. Im using this name because 2 years ago people was doing only 30->60 (x2). Frame interpolation is more correct. ( I personally use 25 -> 75 always.. and that is x3)
About the limitation .. I know that unexperienced users can change parameters and destroy the effect that you have tested.. but at least leave an option to change every parameter for testers and pro users (Im not pro user but I love investigate). A hidden menu or similar.. so amateur users dont need to worry about it.
About levels.. yes.. I know that 0 would be best.. but sometimes with certain scripts maybe you would like to use different values.
For a normal user that does not know about scripts the current easy way is the best. But for avisynth oldies there are lot of limitations if you are not able to modify every parameter.