It's quite a relaxing game, I only noticed at the end that there was a timer! (unless it was scripted?)
In any case, the Key of Souls puzzle is both too simple and too hard: with the pieces available, the combinations are very limited (by looking at where the empty square is on the bottom line, you know where to put the big piece and the other two have very few possibilities after that) nevertheless, if the pieces were more varied or the area where to put them larger, I think it would be far too complicated...
I think it would be possible to counterbalance this: have a larger area, more pieces, and either constant vision and limited time (the more time passes, the fewer features are revealed) or “jockers” that allow you to look at the solution, but the more you use them, the fewer features are revealed.
In short, there's potential!