It will be sunny tomorrow, we are going to go fishing. (you know it will be sunny tomorrow)
Since it will be sunny tomorrow, we will go fishing. (since: you know, therefore)
[It being sunny tomorrow, we will go fishing.] - It being sunny tomorrow, sounds like Since it will be sunny tomorrow
He 'will' be back/He'll be back
She 'will' be going now/She'll be going now
Like how we have I'm for I am and He's for He is
彼が戻ってきます (google translate)
will: want to(for something to happen), for something to happen in the future (future tense)
going to: want to go to (also future tense)
If you think about it, will can be used for more meanings
Going to is directed towards somewhere, a place
