I just cloned and updated the version of ATM10 from v5.5 to v6.1. In doing so, I noticed that curseforge did a full re-download of all mods. You could instead keep a local library of mods that contained files for each modpack that pointed to what mods were within. You could hash each mod file/folder to ensure that you wouldn't be copying over, say ATM's tuned mod A instead of Prominence II's mod A. When downloading, do the same thing. Hash each mod that's coming in (pre-hash the base copy to save time on your end?) and see if you have a matching hash (mod with same version, config, pack-specific tuning, etc.) locally and just copy it over. Would speed up a lot of downloads for people with multiple modpacks already saved.
I realize now I put this in the context of Minecraft because that's what I use Curseforge for, but this could be reasonably applied to most games.