PIG Games first product: Presenter (for C64 OS), is developed fully natively, on-device on a C64. Current development is done using the quite awesome TMP+ (REU version).
This has worked pretty well, but wouldn't it be nice if there was a native, on-device (assembler) development solution for retro/vintage machines that has some more modern features for both the assembler (think along the lines of Kick assembler, 64Tass, etc.) and the editor (think syntax highlighting, multiple file support, perhaps even code completion, etc.).
Today, we have many options of platforms and extensions that can do a little more than a bog standard C64 (or contemporary): Ultimate64 (and U2+L) MEGA65, CommanderX16, SD2IEC, REU (clones), etc.
This means that a development solution can assume, fast assemble speeds, better more structured file storage, loads more memory, higher display resolutions, DMA and such. These could (perhaps even should) enable much more powerful and user friendly development solutions.
So that's why I've restarted work on a project to develop this solution: asm465. It is still early days and progress can be followed for now here: https://github.com/pig-games/asm465 (you can also find more details about the project and its goals there).