Every single alternate incarnation of Finn we see is missing a right arm. The Lich one, Pillow Universe, Shoko, and other extras.
The reason I point this out now, is that if you've been keeping up recently you'd notice that Finn has recently obtained a cursed Grass Sword that seems permanently attached to his right arm.
I'm not saying that the grass sword will be responsible for this, but maybe it serves as a foreshadowing for him losing his right arm.
What do you think?