I felt the presentation of the goal of this game was already a bit confusing, and keeping it simple by just having the player move boxes onto designated tiles would be best.
I honestly do agree with you though. The open ended nature of the slimes contribute less to the satisfaction of completing the level because they usually are just strewn about wherever. I found the satisfaction from just figuring out the slimes to be good enough at the time.
If I expand on this concept in the future, certainly I will add somewhere for the slimes to go. Or at least just make it feel more satisfying