From a narrative perspective, what I want to see is actual movement as opposed to just action. What do I mean by that? Well:
Going by SF Debris’ usage of the terms:
Action: is stuff happening within the narrative, like exposition, setup, characterisation, conflict, et cetera.
Movement: is all the stuff mentioned above [and then some] helping move the overall plot in a general direction.
Bringing it back to FiM, an example of movement was in the first season, with the buildup to the Grand Galloping Gala. Even if only subtly, one of the narrative threads of the first season was the episodes related to the GGG, which gave the series a sense of direction, thus movement, instead of it just meandering about in the characterisation slice-of-life pool.
All three seasons so far had action, and plenty of it, but only season one really had any movement.
The episodes in season 3 before AFR(Apple Family Reunion) seemed to imply movement, but that didn’t really amount to anything in the season finale.
So in conclusion, I want the show to pick up the movement generated in previous seasons (Celestia, Luna, The Crystal Empire, Equestrian Games, reforming Discord, Changelings, Rainbow Dash and the Wonderbolts, Twilicorn, Cadance) and actually go somewhere with it. I mean, slice-of-life is fine, but geez, they introduced/set up all this stuff, so they should at least do something with it.