"We will see"
This sentence really needs an object to translate it sensibly. It'll probably be deleted in the revised course.
Here are the variations:-
Gwelwn ni Dewi yfory = We will see Dewi tomorrow ie:- at some point tomorrow we will see Dewi
Byddwn ni'n gweld Dewi bob wythnos = We will see Dewi every week (lit:- we will be seeing Dewi every week) ie on more than one occasion in the future
Gawn ni weld = We will see (lit let us see)
'Byddwn ni'n weld' is only correct as a question 'Beth byddwn ni'n weld?" = What will we see.