This layout would not allow a quick, unfair win, yet players would be scrambling from the start. One seahorse on the vision square, the other pieces randomly placed in the positions shown, surrounding the target square.
 


 
 
 


 
 

Those weird seahorses would be more effective if there were more of them, more options.
So there is a variety of options for setup. We have 8 cubes, 6 chess, 2 seahorse. There are alternative arrangements, like 8/5/3, 8/6/3, 7/6/3, 8/6/4 but changing the number of seahorses requires a compromise somewhere. Like reducing the number of cubes or chess pieces. Chess pieces other than rook bishop or knight cause problems, and an odd number of cubes would cause more rules for setup. The empty square could have a piece. Extra pieces would mess with that nice setup.
6/3/3? I'm listening. But this would require a different setup and possibly a new board size.
 
 


 
 

Lots of open space for pieces to fly around the board, typically with rook moves.


 
 


 
 

Consider how bishops would work with different board arrangements.


 
 


 
 

Cubes at the front, seahorses in the corners. This solves the problem of random setup (although in terms of game play the cubes would still be random). Has a neatness to it.


 
 


 
 

Looks like familiar chess (it makes me think of Dablot Prejjesne). Solves the problem of random setup. Cubes at the front, seahorses in the corners and out in front. What would tactics be like early in the game? This setup would allow four seahorses. This is the most likely setup.

If you remove the seahorses out front, you lose some tactical colour. On the plus side it would be very clean, also traditional and familiar.