So I would have to hand in my "second best" idea, in order to do what I really want at a later stage, wouldn't I?
No editor (or paying audience) wants to read your "second best" idea. But the editor does want to see that you can follow the strict rules of the guidelines for new writers, to see if you have much more than two ideas in you.
Ideas are the easy bit.
As many successful writers will say, get some mainstream, commercial writing credits behind you before taking the huge gamble of proposing something for a licensed tie-in.