We know that the proper and original meaning of "will" (verb) is, roughly, "want"; similarly, "shall" means "must, ought to".
But "will" and "shall" are normally used as the auxiliaries of the future tense.
(Please note that my question is not, or at least is not directly, about the use of "shall" and "will" in the future: it's about these two verbs in their original meanings of "must" and "want" respectively.)
So, here's my difficulty: if I say "I will go", or "I shall go", that will be understood by everybody as a future; what am I to do, if I mean to say "I want to go" or "I must go" respectively? (Well, you might answer, of course: just say that, "I want to go" and "I must go"! But suppose I want to use "will" and "shall".)
If, in speaking, I pronounced "will" or "shall" with emphasis, or, in writing, I wrote these two words in italics (or underlined them, etc.) — thus: "I will go", "I shall go" —, this would probably be understood as an energetic future, but still as a future (admittedly, in the case of "I will go" this energetic future would often be not very far in meaning from an energetic volition: as in "Whether you agree or not, I will go!" But, even then, this is not the same as "I want to go").
How can I use "shall" and "will" with the meaning of "must" and "want", and avoid every possible confusion with the future tense?