Databases instead of Hypercard...

Another approach for replacing Hypercard might be a database like FileMaker or 4D. While both run on MacOS X and Windows, and FileMaker even has mobile versions for PalmOS and PocketPC, albeit very poor ones, they lack the elegance and familiarity with Hypercard. On the other hand, they are both multi user and as real databases you can throw nearly every computing problems at them.