by-and-by vs time to come

by-and-by and time to come both are nouns.

Word	Noun	Adjective	Verb	Adverb
by-and-by	Yes	No	No	No
time to come	Yes	No	No	No

As nouns, time to come is a hypernym of by-and-by; that is, time to come is a word with a broader meaning than by-and-by:

Other hypernyms of by-and-by include future, futurity, hereafter.

by-and-by (noun)	time to come (noun)
an indefinite time in the future	the time yet to come