adverb
on the way; as one goes
Describes something happening while moving or in the course of going somewhere; often used in literary or narrative contexts.
ゆくゆくは道すがら、美しい景色を眺めた。
On the way, I enjoyed the beautiful scenery as I went.
Commonly used in the pattern ゆくゆくは to talk about future plans or eventual outcomes; neutral in register.
ゆくゆくは自分の店を持ちたいと思っています。
I hope to have my own shop someday.
この経験はゆくゆく役に立つだろう。
This experience will be useful eventually.
いずれ also means 'someday' or 'eventually', but is more common and less formal than ゆくゆく.
将来 is a noun meaning 'future', while ゆくゆく is an adverb focusing on the gradual unfolding of events.
Reduplication of the verb 行く (ゆく, 'to go'), with the sense of 'as one goes' extending metaphorically to 'in the course of time'.