the design is kinda childish.
what do you think?

I like how on the 5x3 there is an empty slot _outside_ the main grid. You park there the final piece to begin with, then solve the puzzle and eventually drop the final piece in for a complete picture. Excellent idea!

I also like the idea of higher order of sliding puzzles. 3x3 and 4x4 need to have some competition!

thank you for the compliment.

I really like the 3x5 and the idea of a hole away from the main square
and the 7x7 is cool but isnt it kind of trivial without adding any challenge? No offense intended

yeah that is right, but the challenge in it is to move the pieces. it locks up most of the time.

What would be is with the 7x7, you put a gap, like on the 3x5, above each of where the centre edges would be on a 7x7 cube. Making four of them means that you could move a piece up to 4 times along instead of just once.