Skal være expresses a planned state:
Han skal være chef for firmaet (He is going be the boss of the firm).
- or a socially constructed fact:
H.C. Andersen skal/skulle være en god forfatter. (HC. Andersen is said to a be good author)
Skal blive expresses something coming up within the plan or course of things:
Han venter på, at det skal blive hans tur til at springe. (He is awaiting his turn.)Jeg er bange for, at han skal blive alkoholiker. (I fear that he´ll end up as an alcoholic.)
Skal (nok) blive expresses a promise or an assurance:
Computeren skal nok blive repareret inden torsdag. (The computer is surely going to be fixed before Friday)