Es wird fantastisch!
The main (and only) verb of the first sentence is werden in present tense. In German, it is quite common to use the present tense to denote the future.
Es wird fantastisch sein!
The main verb of the second sentence is sein in future tense (Futur I, which uses werden as an auxiliary verb).
Thus, both sentences are correct (although the first one is more common), but in the first sentence, werden is the main verb, in the second, it is an auxiliary verb.
Note that this is not an ellipse - nothing is left out, instead these are two grammatically different constructions.