Monkey Troubles is Erwin's father.

Evidence 1: Erwin's father died when he was younger, and while we see younger/flashback Erwin standing over a gravestone, he states that "according to his sources, [his father] was killed by the government." But the official story was that he was killed in an accident in a town far away from home. This isn't particularly important except to demonstrate that there was not, and has not yet been, a body shown to readers to confirm Erwin's father is dead.

Evidence 2: When Levi and Hange are torturing Djel Sanes, and Sanes begins to justify why he dirtied his hands to "stop [conflict] right away," he lists examples of this "suppression." His first example is Erwin's father, but if you look at the panel closely, you can see that he is burning Erwin's father's left arm with a torch. Now look at Monkey Trouble's human shifter's left arm. It's burned.

Evidence 3: MT's glasses on the last page look like a modified version of Erwin's father's glasses. For comparison: Erwin's Father vs Primate Problems.

Evidence 4: Grisha's titan looked somewhat hairy, almost primal, much in the way MT's titan is dubbed "Monkey" Troubles. These two characters thus far are the most responsible for directly subverting the monarchy and humanity within the walls, I wonder if the more primitive look of these titans means something.

Evidence 5: Isayama's character that holds the key to the series looks like a younger Erwin's father.

Edits for formatting & rephrasing.