Formal:
* Hwyl fawr, mi a gofiaf amdanat ti. (This is the most literal translation and uses the formal "ti" for "you.")
* Hwyl fawr, mi a chofio amdanat ti. (This uses the slightly less formal "ti" and is more common in everyday speech.)
Informal:
* Hwyl fawr, mi a'th gofiaf. (This is a more informal way to say "Goodbye, I will miss you" and uses the informal "th" for "you.")
* Hwyl fawr, mi a gobeithio'r gorau i ti. (This literally translates to "Goodbye, I wish you the best," but conveys a similar sentiment to "I will miss you.")
More affectionate:
* Cariad, mi a'th gofiaf. (This means "Love, I will miss you.")
* Mi a gofiaf amdanat ti'n fawr. (This means "I will miss you very much.")
It's important to note that the choice of expression will depend on the relationship between the speakers and the context of the conversation.