I saw you are implementing new primitives such as appending to token list. This is interesting idea and I wander could you implement more new useful primitives like \appenddef, \prependdef where it would add something at begin or at the end of macro. And where you should not care about commands arguments. There is etoolbox with similar commands like \appto, \preto, \apptocmd,... I think primitves would be much more faster and trustworthy.