"This topic is one I've been wishing/wanting to hear about for so long."

Hello! "This topic is one I've been wishing to hear for so long." is almost correct - you just need to add a determiner before the subject "I" and a preposition after the verb "hear" to make the information more specific. So I would write it like this: "This topic is one "that" I've been wishing to hear "about" for so long." We need to add "that" to show that the "topic" is exactly what the subject is wishing to hear about. and We need to add "about" to indicate that the subject wants to hear the information around that specific thing. I hope that helps!