I actually tried doing something like this maybe a year back, but it got out of my scale in terms of complexity vs the amount of time I was willing to put into

I actually tried doing something like this maybe a year back, but it got out of my scale in terms of complexity vs the amount of time I was willing to put into it. Having to manage jar and asset patches, replacements, additions, deletions (as well as the same things for the items within each jar, for example), as well as having to handle config files and other random messes that mods created, without any sort of breakage was quite difficult, especially considering how incorrect installation order of mods would often cause conflicts (meaning that even if modloader and forge, for example, worked alone and forge could be installed before modloader, installing modloader first might break forge). Making this work without an accidentally Turing complete build system is very difficult. Maybe a standard MC build system and API would help clean this all up to the point where Mojang could create a working system to handle all of this properly while interfacing with the launcher's profile system.