Choose to which case of the use of the future simple this example belongs: quick situative decision, future prognosis, question-offer with shall I/shall we.
Shall we watch a film tonight?
We use the future simple in three basic cases:
We use the future simple when we make a decision right now, on the spot, we didn't make plans before.
A: I'm thirsty. B: I'll give you some water.
A: Colin hasn't come to work today. B: I'll call him.
We are speaking about some events in the future but we are not 100% sure that they are going to happen.
"Red Tigers" will probably win the next game. → They might win or they might lose.
Roger will buy a new house in a few years. → Something might stop him from doing that, for example, he won't have enough money.
Sometimes we want to suggest doing something in a polite way. That's where we can use "shall I" or "shall we". Don't forget that we can only use shall with "I" and "we".
Shall I close the window? → Is it alright if I close the window? Do you mind it?
Shall we go for walk? → What do you think about going for a walk now?
Shall I call you at eight? → Let us arrange a call at eight.