(WARNING: SPOILERS AHEAD.)
Attack on Titan Chapter 121 made a startling revelation: that Eren, using pathways combined with the power of the Attack Titan, altered the past and manipulated his father into murdering the royal family though originally Grisha refused to kill children.
So what would've happened in the original timeline? The one in which Grisha walked out of the Reiss chapel emptyhanded? Just for fun here's some speculation:
The following five or so years would proceed more or less the same way in both timelines. The "bubble city" of Shinganshina + the outermost wall would collapse. Eren, Mikasa, and Armin would escape to behind the middle wall. Grisha would still be alive. He would take in Eren and Mikasa, along with Armin after the death of his grandfather.
Because of his personality, Eren would refuse to see reason and join the military. In spite of Grisha's continued survival there's some chance his two young comrades would join him.
Near their graduation Reiner and pals, also cadets in training, would attack Trost.
Eren would get eaten early into the battle, and this time he would stay dead. Mikasa also nearly died in the timeline we know if not for titan Eren's intervention, so she'd probably end up dead too, though she resolved to keep on fighting and so might have made it out.
As they had no qualms about carrying out their mission thus far, Reiner Bertolt and Annie would've most likely proceeded to breach the middle wall.
At this point Paradis's fate would be sealed. Even if the innermost wall wasn't attacked, the survivors holed up behind there would be faced with such a dire food shortage that everyone would be dead in two or three years at best.
If I'm not mistaken, the royal family lived deep in the interior. If so, they'd be among the last killed. But if, say, they were among the first to die as Frieda most likely planned, then the people behind the remaining two walls might've survived as there'd be no pressing need for Marley to continue their attack. The "one surviving human" behind the outermost wall would be whatever titan ate Frieda. Even if that person were then eaten, another titan would become "it". If Marley found whoever was it, then they could bring that person back with them as a prisoner.
Assuming this person wasn't Dinah, Marley would have no way to use this person to kill the remaining Paradisians. However, a sizable chunk of the island would be cleared of possible military resistance, and drilling activities could commence.
However, this is where Zeke, possessing royal blood, would make his move. The end result would be the sterilization of all Eldians, an outcome more or less favorable to Marley since they'd have sufficient time to transition to a post-titan military apparatus before the last of their warrior candidates died of old age.
HOWEVER, what if Grisha had passed on his attack titan power to Eren as in the timeline we know?
In that case, the Warriors' assault on Trost would've failed as before, assuming the "it" person was never found by Marley. Everything would proceed the same until the end of Season 2 as per the anime, when Eren made physical contact with titan Dinah. At this point he, Mikasa, and probably Armin would've been eaten at long last, though Reiner and Bertholt would probably be unaware of this fact (initially, at least). Dinah would end up in Paradis's hands, and she'd be obliged either to continue the fight or pass on the attack titan to someone who was so willing (say, Erwin), assuming they all made it back alive from that expedition to begin with.
And that's all I got.
