I'm hoping that SOny or Microsoft can come up with a software solution for transitioning game files from an external drive over to SSD when the game is going to be played... Like, something like keeping unplayed games (like over 6mos or something when below 20% storage on the internal SSD) on the external drive, and then if you go to launch the external HDD game, it gives you a message, like, "Optimizing Game" or something, and then it moves it onto SSD and moves another game to your HDD and then launches. I kinda expect Microsoft to build something like this sooner than Sony. It's easy to do it manually but would be nice if it happened automatically, or even if the UI gave you a notification like "We've noticed you haven't played these 4 games in X months, would you like to move these to external storage?" You confirm, and then they automatically move those games over, and if you go to play one of those games maybe you have to wait a few mins for it to transfer back to internal storage... Perhaps with some features able to launch immediately.
It won't solve the problem but there will be a bottleneck with game file sizes and SSDs still being pretty expensive.