Microprocessor Design/Compiler
With an assembler written, it is typically a good idea (although not always) to write a high-level language compiler for your new processor. Typically the high-level language in these situations is C because of its small number of built in constructions, and the close relationship that C shares with the underlying assembly language.
Compilers help to speed up the development process, so that more complicated software can be written without the tedium of writing large assembly language programs. Another benefit to this is that there are a number of pre-existing tools for use with higher-level languages, such as simulators and debuggers that can increase the efficiency of your software team.
Further reading
edit- Compiler Construction discusses how to write a compiler from scratch