There is one CD-i prototype known of the unreleased project 'Mind Quest' - thanks to CD-i member Seventy7 we can see some video footage of it as well as various screenshots below. It was developed by Fit Vision (The team is nearly the same as the team behind 'Great British Golf' and 'Nature under Threat' and published by Philips Interactive Media International.
This game remained unreleased on CD-i. Our prototype is a 'Testing Phase 1' disc, version 1.02 (so the third testing disc they prepared) dated January 24th, 1994. It holds various minigames and puzzles and it is a base-case standard CD-i disc, no DVC required.
The game however was reviewed in USA CD-i Magazine Issue 2, rated 70%. You can download an early prototype dated August 3rd, 1993 from Hidden Palace and Archive.org. The available download works in CD-i Emulator 0.53 beta 4 or higher (no sound issues as reported before, but it depends on your setup).
Mind Quest is some sort of Brain Training game on CD-i, offering you all kinds of brain games to stimulate your ability to solve, memorize and think logically. It was reviewed in USA CD-i Magazine issue 2, but somehow it was never released in the end. The reason for this is still unknown. The game plays very well and it looks like a complete title, we didn't find any bugs in it, but perhaps it depends on the type of player you are testing it on.
It is a base-case CD-i title, you don't need a Digital Video Cartridge. There are crossword puzzles, sliding picture puzzles, mathematical puzzles, visual puzzles and all kind of logic puzzles to solve. It is a collection of minigames. There are three difficulty levels and the characters in the game even give you feedback on how you perform. There is a high score table so you can improve on your score every time. Navigation can be a bit confusing but the execution is very nice. It is a pity this game was never released.
Find below the game in action, courtesy of Seventy7 from the excellent Preserve CD-i Blog:
[Thanks, Seventy7]