Chaos and Cards Part 1: The beginning of the adventure. Sonic has to be around Sakura's age, that being ten years so they can relate and help each other out as well as ideas for the team dynamics Sonic being the skills and speed, Knuckles the Echidna is the muscle, Tails the brains, Keroberos the guide to the cast, Syaoron a reluctant ally who ends up helping to teach Sakura and the Sonic Cast paths of magic with Sakura also branching out from just her Clow Card skills, and Sakura being the heart of the group. Imagine Sakura gaining all her cannon powers and also using Chaos Energy and the Sonic Cast learning magic along side learning how to use Chaos Energy unlocking their full potential and all their cannon powers!

Chaos and Cards Part 2: The Sonic and Card Captor's Systems of Mysticism.
If you go with the games the Master Emerald was created by the Gods to keep them in balance which truly has infinite power and in the games exists as a universal singularity as, their is only one Master Emerald across time and space, The Chaos Emeralds have potentially limitless power while the Master Emerald truly does have limitless power. From what I know about Chaos Energy and the Chaos Force with the Archie Comics on that topic, Chaos energy is a product of the Chaos Force, a never ending pool of Primal Cosmic Energy that is tied to the very universe itself with the Chaos Emeralds being the strongest physical embodiment of Chaos Energy in the Pre Super Genesis wave continuity in the Archie comics but can also be drawn upon with certain technology and mystical relics and mystics. After the continuity reboot with the Super Genesis Wave, The Chaos Emeralds have potentially limitless power while the Master Emerald truly does have limitless power like in the games. If you count Sonic 06 they can even cause Perfect Resurrection something only Clow Reed was able to do. Both aspects of the Chaos Emerald in both continuities are merged and the Master Emerald is the one from the games in this story.

The Magic's of the Cardcaptor Sakura series revolve around a mixture of actual eastern and western mystical systems and iconography utilizing the power of Celestial Bodies and the Elements along with cosmic concepts like Chaos which you can use to merge both universes systems of Magic. In Sonic 3 and Knuckles when going for 100% Completion you see a mural in the Hidden Palace Zone with Super Sonic and Eggman in combat, and in Sonic Adventure 1, and 2, you see that Sonic is implied to be something more than just the world's fastest hedgehog. If you decide to do this please give Sonic and his team all their gear and equipment and powers from all the games and all the powers from the Archie Comics and all across the Sonic franchise. Both of the franchises do have mystical elements and both Sonic and Sakura are sort of Guardians of mystical artifacts, The Clow Cards for Sakura Kinomoto, The Chaos Emeralds for Sonic as well as The Seven World Rings, The Super Emeralds, Time Stones,(the list goes on)and The Master Emerald for Knuckles the Echidna. Imagine if someone with Mystical knowhow had helped Team Sonic use the tools at their disposal and their dangers along with Team Sonic helping Sakura grow into her role as a Cardcaptor. Also the Sonic crew has faced mystical and cosmic antagonists before like Perfect Chaos, Dark Gaia, Mammoth Mogul, Enerjak, Solaris, Mephilis the Dark, Iblis, Erazor Djinn, Merlina, and their are other magical artifacts in the Sonic Series such as the Sol Emeralds, The Jeweled Scepter, The Crystal Ring, The Phantom Ruby, The Swords of the Knights of the Round Table, Caliburn, The Chaos Emeralds, Power Rings and Super Emeralds obviously and other Mystical artifacts and powers like Mystical Melody, which most of the Sonic cast can use. Their are even other mystical entities on the good guy's side like Light Gaia, The Ancient Walkers and their successors the Neo Walkers(these members are Aurora, Athair, and Merlin Prowler, the uncle of Tails), Lumina and The Dream Goddess, Tikal, Sharrah, Athair, Chaos when he gets his act together, and The Brotherhood of Guardians.

Chaos and Cards Part 3: Parings. Now if this idea gets off the ground seeing as Romance plays a massive part in the Card Captor Sakura series Sakura Kinomoto has to hookup with Syaoron, Sonic with Amy, Knuckles the Echidna with Rouge the Bat(when he gets older), and Tails with Cosmo, or Cream the Rabbit.

Chaos and Cards: Part 4. The World of the Story and the Series Storylines. Now if you go with the Sonic X Anime as a part of the crossover, Sonic and his friends along with Sakura Kinomoto have to go through the events of the Sonic X Anime and the Card Captor Sakura anime including the Card Captor movies with elements from the Sonic Archie Comics Pre Super Genesis reboot for the world that Sonic and his friends came from and for the mystical elements and the powers of Chaos Energy and the Chaos Force for the Sonic world's mechanics and for the building of Sonic's world and go down the events of the Sonic X Anime with some elements from the comics. Some of the stories from the games and those in the comics including spinoffs like Sonic Universe and other side comics like in Knuckles the Echidna can be implemented in the Sonic X anime but only if they fit with the Card Captor Sakura mystical elements or the Sonic X Anime story lines.

Now if you go with the Sonic The Hedgehog games for the crossover the world of the Sonic games and the Card Captor Sakura series are merged with the Sonic Adventure and Sonic Story Book series and finally Sonic Unleashed and Sonic Generations being the endpoint of the Sonic portion of the crossover and the Card Captor Sakura movies the end point of the Card Captor Sakura series portion of the crossover.