Some modpacks are BIG, like hilariously unwieldy to an absurd degree, this results in 2 things, lots of modpacks take up lots of space, and some especially large modpacks, take forever to load on HDD (upwards of 20 minutes, ATM 6 I'm looking at you).
Most people I know, have both an SSD and HDD, and it would be nice to be able to keep the vast majority of my modpacks on my HDD, to save space on the inevitably smaller SSD while some, that load painfully slow otherwise, on my SSD.
Obvious way would just be to rip steams system, and make a way to add a separate location on the other drives, and then just pick between the drives.