Janus & Schala?
If you cast Black Hole on one of the Chronopolis islands, an image briefly flashes in the sky appearing to depict a young woman, a boy (or girl), and a wooden art figure. There's no real reason for this image to be here, leading some to speculate it's an easter egg depicting Janus and Schala. The purpose of the art figure is totally unknown.
Thanks to Molotov for the image
Magus was originally planned to be in Chrono Cross as Guile, but this was scrapped after the designers realized his story would be too difficult to integrate; within the game, he is only referenced as Janus in a letter from Lucca to Kid. Magus is rumored to appear in the form of a shadow during the reading of this letter. This appearance is consistent with his abilities in Radical Dreamers; in that game, he was described several times as a shadow walker and demonstrated the ability to recede into the shadows at will.
Ultimately, the shadow is supposed to be generated by Luccia's letter to Kid:
Radical Dreamers is mentioned sort of as a separate dimension in Chronopolis. Specifically, a terminal in one of the upper labs plays the opening dialogue of the game.
[Computer] ~~~Read the report ~~~Quit It all began with a notebook my mother handed me... She told me she found it while cleaning out the closet. This old notebook, covered with mold, was a diary that belonged to my grandfather, who passed away years ago. It contained my grandfather's distant memories... Member: Does this device playback archives? Member: Looks like someone's memoirs. [Computer] ...... I wonder if you still remember... How we first met... and our many adventures... It all seemed like a dream... We ran like the wind during those warm, summer days...long ago... You were a piece of a star, that fell from the sky... I can still relive the memories of those days long past... Just by closing my eyes like such, and whispering your name into the desolate night... Kid... ~~~~~Kid... Radical Dreamers - Le Trésor Interdit - Kid: Radical Dreamers...!? And me name's on here, too! What the bloody hell is goin' on? Kid: And here's a passage with me and me old mates. Member: What's this...!? Member: Radical Dreamers...!? What does this mean? Member: There's a conversation with a comrade. [Computer] Are you ready, Kid? I know you're anxious, but stay on your toes. Yeah, likewise, mate! You mess up, and I'm leavin' you behind! Got that, Serge? C'mon, Magil, let's go. That bastard Lynx is gonna get what he deserves! Say your prayers! ...Not that it'll do ya any good!!! Heh-heh-heh...... HA-HA-HA-HA-HAAA! Kid: ...... This seems to be an archive from a different time than our own. Kid: Aside from the two worlds we already know about... there may be other worlds and times which exist... Member: ...... This seems to be an archive from a different time than our own. Member: Aside from the two worlds we already know about, other worlds and times may also exist...