Okay so I have a theory... It is based entirely on speculation and has zero circumstantial evidence from either the anime or the manga... But the more I piled onto the theory the more it made sense in my head, at least. So, I thought I'd share it here. Be advised that said theory will contain spoilers.
So I was just making dinner when I said to myself as a joke, "What if Ymir's last name is Jaeger?" But the more I thought about it, the more it kind of made sense.
The only two people in the series thus far that weren't given last names right away were Levi and Ymir. We eventually got Levi's last name, and why was that kept a secret for so long? Because his last name is the recognizable Ackerman. What if Ymir's last name is still a secret because it's the recognizable Jaeger?
In Ilse's Notebook, that Titan says "Ymir-sama, Ymir's people," which could imply that Ymir was some kind of royalty. As we all know, it is stated that only royalty can utilize the full power of the Coordinate, and Eren (while not having mastered the Coordinate so far) has shown that he is at least able to utilize it somewhat.
Eren's father came from beyond the walls, so it's not like it's absolutely absurd to think that they might be related somehow.
Okay, it most certainly is absurd to think, and the theory is probably a load of garbage.
But I just thought it was neat that a lot of these little things meshed together.