the verb aller normally requires an object such as the adverbial pronoun y meaning there. The word there can often be omitted in English, but y usually cannot be omitted in French. Je vais (I'm going) is not a complete sentence in French; if you don't follow the verb with a place, you have to say J'y vais.
Il faut y aller - we must go
allons-y - let’s go
However, there are exceptions such as for conjugations in the simple future. tense. In the simple future , the conjugation of aller starts with an i (iras, iras, ira, etc.), so you cannot use y as a destination otherwise you will have two consecutive vowels that clash. Therefore, either you use "j'irai là, j'irai là-bas" or by exception, you can use j'irai, etc. by itself.
il ira seul - he will go alone
Elle ira le voir. - She'll go and see him.
Allons y.. on y va.. j'y vais.. I've seen and heard all three of those phrases used without a destination mentioned. In french if you're not staying then you have to be going somewhere which is "y". So the "y" can be non specific or a prevously mentioned place so you dont sound like a wierdo saying the same thing/place over and over. I thought there should be a y in this sentence but im still learning too.. high probability that i'm wrong.
What is wrong with "everyone except you will go"? It looks like perfectly acceptable English to me and means the same as "everybody will go except you. I would say that my version is better English, correctly linking, as it does, 'except you' to everyone rather than to 'will go'.