With Finn defecting from the FO, you get a short glimpse of a storyline where Stormtroopers get humanized (is that even a word?) and that Finn might play a role in creating a revolt against the FO leadership, helping the Rebellion... but nope.
Sad thing, too, it would have made for an amazing plot and practically writes itself. Here, let me have a go:
First, you retcon TR-8R's death from TFA to him being heavily injured and have him be Phasma's henchman who then faces Finn in the next movie. Explore his and Finn's history together and maybe how Phasma figures into all of this. TR-8R is Finn's former friend from the same village, who has been indoctrinated into the whole FO shtick to a degree that he can't deal with Finn betraying the FO.
Have a flashback to Finn as a kid, we see an attack on his village that was (apparently) committed by the New Republic for some political reason (like, the village is on a planet that wants to remain neutral and seperate from the Republic). Finn and his friend flee the massacre of the townspeople, but Finn gets seperated for a short moment. As he's hiding, he watches a soldier removing his/her mask (Could be Phasma before she got to a leading rank, to tie her into the plot a bit more) or helmet for a quick moment before leaving. Finn then stumbles through the ruins with his friend after the attack until they are eventually picked up by FO-soldiers that "rescue" them. In another flashback, we see Finn (much later and during his training to become a Stormtrooper with his friend) notices another Stormtrooper, who looks very similar to the soldier he saw during the attack. We learn that it was a false flag by the FO, to obtain a few easily brainwashed townspeople as slaves and a few children to indoctrinate in their military structure. Finn then slowly learns about the full extend and this builds his desire to defect. He tries to tell his friend, but he's not receptive to this stuff at all due to indoctrination and Finn is punished and re-educated (which is also why he was demoted to being on "mop the mcguffin-room" duty).
This would all lead up to a big climax between TR-8R and Finn, where either Finn is forced to shoot his former comrade or he actually manages to pull his former comrade on to his side. I would lean towards the latter, tbh, since that could be used for a pretty epic scene where Finn is captured and about to be killed, but TR-8R shows up and rescues him. Or he takes him away to be "interrogated" and brings him to a group of other defectors who are still with the FO, but prepare for a large scale uprising.
This is stuff that I came up with as I typed and it would have made for an absolutely amazing story arc for Finn.
What do we get instead?
A retread of Finn's arc of TFA and... this scene:
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