Both Latenite and Brass are getting a significant upgrade - and both are being written from scratch.
Both will sport a plugin-based architecture. This is most obvious with Brass - where pretty much everything - be it an assembler plugin or output plugin - can be extended by writing your own custom plugins. All Brass does is parse the basic syntax and pass it to the various plugins to work out what to do with it!
Latenite will load Brass and use it to provide feedback - such as error reporting and syntax highlighting - directly to the user.
General discussion is handled here; for what I mean with regards to Brass plugins, there's this post.
The basic idea is that you can plug in your own assembler and use it alongside Brass and Latenite.