Would-be in a sentence

The example seems to me to be a modern attempt at using archaic language. The OED contains no note of "would-be" in this meaning and I doubt that it has been used in that way.

Although in the context, "would-be" can be easily understood as having a future intent - "the woman who would be my wife" - "would-be" is a nominalised adjective rather than a pure noun = my would-be wife. The commonest current form would be fianceé, - a French past participle adopted as a noun - i.e. the person to whom you are engaged to be married.

Would-be is a combination of the simple past of the verb "will" in its sense of to want; to wish; to intend or shall/should like, and "to be".

A commoner, and informal, nominalised adjective, also verbally derived, for such a woman is "intended", thus: "Wedding songs turned dirges when my slain intended was taken to her resting place."

"Would-be" as an adjective is very old:

OED A. adj. a. Of persons: That would be; wishing to be; posing as.

1300 Graystanes in J. Raine Hist. Dunelmensis Scriptores Tres (1839) 77 Eum [sc. Henry de Luceby] contempserunt, vocantes eum H. walde be Priur. (Him [sc. Henry de Luceby] they had scorned/mocked, calling him "Henry [the] would-be Prior.")

1864 E. B. Pusey Daniel ii. 91 Antiochus was a propagator of false religions, a would-be destroyer of the true.

1889 F. E. Gretton Memory's Harkback 307 Napoleon I..actually bequeathed a legacy..to Cantillon, the would-be assassin of Wellington.

Its use as a noun (a person who would-be) is also old, (but not that old) and usually carries various levels of negative nuance - someone who wants status or social position but whose limited talents will mean that that is very unlikely to be a reality, or someone who is simply aspiring (realistically or not) to that skill or position:

OED

B. n. One who fain would be (something specified or implied).[...]

(b) 1672 A. Marvell Rehearsal Transpros'd i. 238 They are the Politick would-be's of the Clergy.

c1730 A. Ramsay To his Son vi Yet, this let little would-be's know.

1732 London Mag. 1 240 Of all the Fops in Nature, none are so ridiculously contemptible as the Wouldbees.

1782 W. Cowper Conversation in Poems 243 A man that would have foiled at their own play, A dozen would-be's of the modern day.

(Compare the current, informal/colloquial noun and adjective "wannabe".)