Firstly, allow retry on failed downloads.
A lot of times if curseforge servers are under heavy load, they can drop download chains, causing people to have to restart their downloads. This also causes more load to the servers.
Hash mapping / unique downloading only.
When updating a modpack, rather than deleting all the mod files, we should check the hashes for the ones in the folder, and the ones that are in the updated version, so that we don't have to delete the entire folder if we'll end up downloading 60% of the same files.
This would also decrease server load.