[gnodar]

Looks like a less feature-full version of livecode [0], which I'd argue is more of a HyperCard successor", since they were formerly revolution & MetaCard, and can import HyperCard stacks.

[0] https://livecode.com/

[jhbadger]

There is also Decker (https://github.com/JohnEarnest/Decker) that is open source and feels much more "Hypercardy", although the retro dithering asthetic may put some people off.

[hyperhello]

Don’t forget https://HypercardSimulator.com which includes all the content from the eighties and nineties too.