But do you think it would be possible to achieve similar results without a new language, but with a declarative API in one of your existing languages (say, C++) instead?
If possible, that would remove a big adoption barrier, and avoid inevitably reinventing many language features.
A dedicated compiler allows us to trace dependencies between state variables and DOM nodes at compile-time, generating direct imperative update code. To achieve similar ergonomics in a C++ library, you'd effectively have to rely on runtime tracking (like a distinct Signal graph or VDOM), which adds overhead.
All the best with it!
P.S. It would also be interesting to see how far it's possible to go with constexpr etc.