I'm planning on doing a bigger write-up on this cause I think it's an interesting approach. Anyway.
Taking the introduction video and ending video, namely, the parts involving the journals, as a whole, it's... basically Kid reflecting on the events that happen in Chrono Cross. To the extent that I wonder if maybe Chrono Cross IS supposed to be the contents of that journal.
Couple this with the ending scene and ending song and the last bit of text about Kid searching for Serge, even if they might not recognize eachother. Saudade and sehnsucht are similar concepts to nostalgia, and they mean, more specifically, fondness for something lost or unattainble, and the search for something that might be unachievable, but there's some sort of fulfillment in the search. The lyrics of the ending song, translated, seem to carry a similar theme.
There's also a strong sense of reflection on the past in CC, in general. Serge and Leena's early dialogues on the beach, for instance, or pretty much everything involving Fargo or the Acacia Dragoons, the recurring theme of mementos and keepsakes, or, most obviously, the way that the entire multi-universe thing plays into the idea of seeing how things might have turned out otherwise.
Finally, I think you can also see Serge, in addition to being the arbiter of time, as a sort of change agent that rekindles some sort of... youthfullness or sense of adventure in the people he encounters, hence them all deciding to join up with him.
Radical Dreamers... I haven't played much. Just started recently. But it seems to have similar themes too.
Also, the campfire scene in CT. I wonder how relevant THAT might be, assuming nostalgia is an important theme in CC. I mean, that entire bit is about the Planet looking back on its life, and the characters wondering if they'll do the same in the future.
So, yeah. Whatever. Any thoughts? Contributions?