Goodbye forever

Solution 1:

I think "Farewell" is the best you can do in English. It's a blessing for a good future and it carries a lot of emotion. "Goodbye" started out as a blessing, but now it usually means "'til next time" and it's a polite way to take one's leave. You might say "Goodbye" to your wife when you leave in the morning, but if you said "Farewell" she would wonder whether you were ever coming back.

Solution 2:

Besides Farewell which seems to me the more appropriate, how about So Long, au revoir (French, but you might see it in English as well), adios or adieu?

In some movies when two characters know that they won't meet again, they say Later, which is a hopeful kind of goodbye, because it implies that you might see each other again (even though you won't)

If you are best buddies with the departing person, you might use an informal word like cheerio.

Or you might turn farewell into a sentence like I fare you well.