That's actually a really good idea.There should be an option to automatically offload games from the SSD to the external HDD if you haven't played it in X amount of days, that would be awesome. You wouldn't have to manage much, and if you want to play something from the HDD, a small transfer to the SSD and you're good to go.
Oh, you know what they REALLY need to do (or at least give you the option)? Always have every game on your HDD. Like, when you "move" the game from HDD to SSD, copy it to SSD but don't delete the HDD copy.
If you don't do that, when you want to make space on the SSD you will have to (1) wait for the SSD copy to write to the HDD, and then (2) wait for the HDD to transfer the new game to the SSD.
But by keeping all your games on the HDD at all times, you can just erase the game you're "moving" from the SSD (without needing to wait for it to write to the HDD), and immediately proceed to transferring the new game from the HDD to the SSD.