I don't speak French, but if the cognates are similar to Spanish, I would say, "Gateway to imagination." EDIT: if this dictionary is accurate (https://www.linguee.com/french-english/translation/port%C3%A9e.html), I would say: Scope of the imagination (which is like the continuum of imagination) Reach of the imagination (which is to like stretch of the imagination) Scope would refer more to limits of the imagination while reach refers more to the extremes of the imagination.

I don't think there is a word. maybe - within his imagination as far as he could imagine what's the full sentence?

Merci beaucoup de ta réponse @Berze. Je pense qu'elle répond parfaitement à ma question!

"within imagination's reach/grasp"

We can make a good comparison with a similar french expression which is "à portée de vue" that we can translate by "within sight". But I was not sure the same approach is possible with the word imagination which is more figurative.. "within imagination" ? Does it look weird for a native english speaker ?