* E'er: This archaic word is often used in poetry for a timeless, eternal feeling.
* Aye: This word carries a strong, affirmative tone and is often used in poems about unwavering commitment or belief.
* For: While not a direct synonym, "for" can be used poetically to convey a sense of enduring time or purpose.
Ultimately, the best word will depend on the specific poem and your intended meaning.