AmigaOS 5 was supposed to be a hosted environment that could work on top of another operating system or an unhosted native version. (According to the now blocked Q&A session with Fleecy Moss on AmigaWorld.net .) It would work on any processor and allow cross-platform development for other operating systems but require the native OS for maximum performance.
My thoughts: Not needed any more now that Java 6 is out and open-sourced. Now we just need an OS 4 version of Java, a decent web browser, and new hardware to run OS 4 on. Also, SDL is very cross-platform compliant but still requires a recompile of the source to work on other platforms. If a build system based on Java or LLVM could make SDL apps binary compatible between platforms we'd have something.