new ballgame vs ballgame

new ballgame and ballgame both are nouns.

Word	Noun	Adjective	Verb	Adverb
new ballgame	Yes	No	No	No
ballgame	Yes	No	No	No

As nouns, new ballgame and ballgame are synonyms defined as:

new ballgame and ballgame: a particular situation that is radically different from the preceding situation

new ballgame (noun)	ballgame (noun)
a particular situation that is radically different from the preceding situation	a field game played with a ball (especially baseball)
	a particular situation that is radically different from the preceding situation