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After re-reading the manga, I'm starting to think that the "something that's yet to happen" is Eren controlling Dina to kill Carla
Grisha started acting weird after he learned Carla got eaten. Before that, he was only searching for his family to make sure they were safe, and not to make Eren inherit his titans. He only started talking about it after learning (or having the confirmation?) Carla died
I don't see the point of keeping that memory a secret if the reason why Grisha changed his mind is learning his wife died and his hatred coming back as a result, because we already know it. But considering he mentioned avenging Carla twice, it's still related to it
And I think chapter 130 might explain how. In 130, we saw a memory shard of the moment Dina ignored Bertholdt after he broke the wall, something we already saw in chapter 96. While you can argue Dina ignored him because she's abnormal, a lot of things are still very odd
First, Dina ignoring Grisha bc of her promise to Grisha makes no sense because if that was the case, she'd have stayed near the walls instead of following Grice immediately after being transformed. Also Eren is the one with Grisha's blood, and yet she didn't go after him
Not only that, but the way Dina killed Carla is weird too. Remember how Armin guessed Annie was a shifter? As he said, all titans do is eat humans, they don't kill for the sake of killing. And yet here Carla was killed BEFORE being eaten, and it's even more obvious in the anime
And to finish... at this moment, Eren saw the future and yet, Dina ignoring Bertholdt is something that happened in the past. There's another memory shard that technically happens in the past (Grisha killing Frieda) & we know what happened: Eren visited this memory in the future
If we follow the logic, then Eren seeing that memory from the past means he's gonna visit Dina's memories of the day his mom got eaten somewhere in the future, and considering Dina's behavior is really weird, then it's most likely because she was controlled (paths transcend time)
And I think this is the memory Grisha saw: Eren killing Carla by controlling Dina.
Grisha told Eren he had to learn to control this power to save Mikasa Armin and everyone else.
But what if "this power" isn't the Founding Titan but the Attack Titan?
Grisha told Eren he had to learn to control this power to save Mikasa Armin and everyone else.
But what if "this power" isn't the Founding Titan but the Attack Titan?
Adult Eren forced young Eren to follow a certain path, making him a slave of the story, fate and himself.
But after gaining those memories of the future, it stopped, and now, he has a combination of powers that allow him to change the past if he wants (and he did in 121)
But after gaining those memories of the future, it stopped, and now, he has a combination of powers that allow him to change the past if he wants (and he did in 121)
There's only one way to change the past and save Carla: young Eren taking a different path than adult Eren, refusing to use his power to control Dina and finally breaking the loop. This is why this message exists, to try to make Eren follow another path.
And I think this is why Grisha gave Eren his titans despite what he told to Zeke. Zeke can save the victims of the rumbling but he can't save Carla, only young Eren can by refusing to follow the corrupted path his older self did. Pretty sure Grisha is talking to adult Eren here
tl;dr The memory Eren showed to Grisha to convince him to turn his younger self into a titan shifter is him controlling Dina to kill Carla. Grisha did it because it was already too late to save Carla, only young Eren could, by taking another path than the one his older self did
And in my opinion it will be a complete fail 
