The verb in Mark 16:16 translated "shall/will be saved" is σώζω (sózó). The form found in this verse is σωθήσεται (sōthēsetai) which is Future Indicative Passive - 3rd Person Singular. [This is a common form of what is technically known as the Divine Passive" meaning that God is responsible for the action, as is obvious here; but I will not discuss this here.]
The same tense of the verb is used for κατακριθήσεται (katakrithēsetai) = "will be baptised".
The tense of these two verbs is a simple future indicative passive, "will be saved". It is not imperative (eg, "must be … "), but a simple future. The subtle distinction in English between "shall be" and "will be" (if it exists at all in most cases) is not present in the Greek tense unless this is interpreted as the difference between indicative and (say) imperative. In any case, "shall be … " is often used in English to indicate a simple future as well, that is, "shall be … " and "will be … " are often equivalent.
In any case, most modern English versions correctly have "will be saved" but a number retain "shall be saved" and in common speech, they are equivalent.