- Multiple languages supported, including JS, Java, and Python.
- Good cross-language integration: inheritance, type matching, etc.
- Cross-language debugging, ideally including C++.
- One GC to rule them all, preferably one shared GC, not a super-GC ruling a zoo of heterogenous GCs and reference-counting subsystems.
- Decent JITed performance, because performance matters when you can least afford to rewrite in C++.
- Sandboxed execution security model, type safety, defense in depth.
- Part of a larger open source platform . . .
- . . . that has momentum and excited developers hacking on it.
- A sufficiently open specification process for standardizing and evolving that platform.
- New XML language implementations easy to write, plug in, and demand-load (if we had this today, we would use it for non-web standards such as XForms).
The last item means some number of frozen APIs into the content code, e.g. XTF. SVG and existing XUL and HTML widgets may provide a rich enough rendering vocabulary. If we can mix them well using XBL, then we may not need to expose frozen APIs into layout, gfx, etc.
The goal should be minimal, supportable, and composable high-level APIs, where possible. XBL and XTF point the way.
Back to the more programming-language-specific points above: comments welcome on realistic candidates. Obvious and controversial possibilities include Mono, Java (if open source), and Parrot (last I looked, *way* too Perl6-focused, not very mature, not looking likely to mature quickly).