Learn Dutch: Wat ga jij in de toekomst doen? - What are you going to do in the future?

When you use gaan to talk about the future, the main action verb gets pushed to the very end of the sentence in its full dictionary form (the infinitive). Here, the helper verb ga takes the second spot in the sentence, and doen waits until the very end.

Why is the verb ga missing its -t? Shouldn't it be gaat jij?

When you turn a sentence into a question by swapping the verb and the subject jij (or je), you must drop the -t. So the regular statement jij gaat becomes the question ga jij?.

Could I use zullen instead of gaan to ask about the future here?

You could, but gaan is much more natural for this sentence. Dutch uses gaan to talk about personal plans and intentions. You use zullen more for making promises or predicting things that are outside of your control.

Why use jij instead of the unstressed je?

Both mean you, but jij is the stressed form. Using it adds emphasis, making the question mean What are you going to do? (perhaps implying that the speaker already knows what they themselves or someone else is doing).