What exactly does the phrase "there may be hope for you yet" mean?

It’s a loaded compliment. It implies that you haven’t given much cause for hope to this point, but that whatever you’ve just done or said may be cause for renewed hope.

It means you are a pretty hopeless case, but you have just demonstrated the possibility of not being hopeless. So there may be hope for you!

But pretty much it’s a backhanded way of saying you are worthless.

I agree with @thorninmud. It’s like the phrase from My Fair Lady, “By George, I think she/he’s got it!” – a celebration, but celebrating the fact that someone who was thought unteachable has finally learned something.

Depending on the tone of voice used, it might be negative, but more often, people use it as a light-hearted jibe. I wouldn’t automatically take offence from it.

I’ve only been told that in a light hearted, funny way.

I agree with @Dutchess_III. It’s a humorous way of giving a compliment, said jokingly.

“By gollly, there may just be hope for you yet!” I have a friend that says that nearly every time you do something even mildly out-of-the-ordinary. He’s a pipe fitter and worked on the bathrooms here lately. When we got done with my shower, the stream was really weak. I unscrewed it, threaded the head back on, then tightened it only enough to keep the water from leaking out of the joint. Strong stream! He said ^^^^

It’s a much kinder way to say a person is not as dumb as they look, sound, or the best testing would indicate.

You are a lost cause. But perk up! Murphy’s Law works both ways, for good and bad, so you just might get lucky!