This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Mario & Luigi: Dream Team
Kylie from Partners In Time is surprisingly back in this game, however she doesn't remember having met the Mario Bros. in the past, it's kinda sad due to the nice friendship between Kylie and both sets of Bros. being sent down the drain.
However, in Partners In Time, they had met Kylie in the past time period of the game, and she doesn't seem to have shown any signs of aging in Dream Team. And it's a well known fact by this point that Koopas don't exactly look all that different from each other. With this info, we come out with the possibility that the Kylie in Dream Team may actually be a descendant of the one in Partners In Time.
When you go into Dream's Deep, you see Luigi's innermost thoughts swimming in the background with Mario on his own whilst Dreamy Luigi is trying to find his way to the place. That onimous music, dark background and lines like "Don't leave me, bro!", which is also sent out in massive letters, really makes you feel for the younger brother. Especially for players who are younger brothers themselves. This gets worse when you realize that Luigi is suffering from a nightmare during this sequence.
Even though it was Played for Laughs, Britta's "death" might have initiated one, as she looked like she wasn't going to make it.
What some of the Pi'illos must be feeling. They were turned into stone pillows for a long time, and finally freed only to find their kingdom is gone, they now only exist in tales and legends, and their island had become a tourist attraction. Nothing would be the same for them anymore.