I am looking for an adjective which could work with the "end of the story", meaning predictability, shallowness. Now I am deciding between "flat" (in Freedictionary 5th,6th, and 12th explanations work well) and "plain" (because it helps to transfer the obviousness, simplicity). Unfortunately, I can't really see the difference between the two synonymes. What would you advise to focus on?
Of the two words you are considering, I would choose flat.
As in: Writers Relief
How To Write A Happy Ending That Doesn’t Fall Flat
I would recommend you use the word "banal" which means:
lacking originality, freshness, or novelty.
Drake hit us with a 25-song-long album last night, and while discussing the length of albums is as banal as talking about the weather: Ye’s was so short!
— Allison P. Davis, The Cut, "What Songs Should You Listen To on Scorpion?," 29 June 2018
The more banal, the more commonplace, the more predictable, the triter, the staler, the dumber, the better.
— Don DeLillo, Mao II, 1991
You might also consider the adjective "pedestrian," which means: