Chrono Cross is a RPG developed and published in 1999 by Squaresoft for the original PlayStation. As we can read on Wikipedia, Square began planning Chrono Cross immediately after the release of Xenogears in 1998. Chrono Trigger’s scenario director Masato Kato had brainstormed ideas for a sequel as early as 1996, following the release of Radical Dreamers.
Developers brainstormed traits and archetypes during the character-creation process, originally planning 64 characters with unique endings that could vary in three different ways per character. As production continued, the length of Cross increased, leading the event team to reduce the number of characters to 45 and scrap most of the alternate endings.
More info about the Chrono Cross beta can be found in the Chrono Compendium:
Before the official release of Chrono Cross, Square Enix marketed a PlayStation release of Chrono Trigger and attached a demo of the game to it and Legend of Mana.[…] This demo has allowed a short look into the late stages of the development of Chrono Cross, offering a few prizes such as a scrapped facial portrait of Kid and missing playable characters.
Serge’s Swallow is tinted green and looks like the Mastermune.
Once the Fort Dragonia sequence is concluded, Serge wakes up in Arni and is instructed to find Leena as usual. Una has no portrait, hampering the suggestion that she was once planned to be a playable character.
The most glaring difference is the presence of a seemingly new character in Pierre’s slot. Since Pierre’s dialogue icon was already in its final state, it’s probable that this was an earlier design for Pierre.
There’s a different Viper Manor Study window.
There’s a different Lucca drawing.
Yasuyuki Honne is the artist of the Chrono series, and made some background art that was never used in the final game.
There’s also an incomplete debug room in the beta demo and some unused characters can be seen in the Chrono Cross artbook.
Also, GlitterBerri translated the Chrono Cross Ultimania interview in which we can read about some ideas that were never used in the final game. You can read the full interview at BerriBlue or in the Chrono Compendium.
Thanks to ace.dark and Megalol for the contributions!