2 spooky 4 scary
As far as randomization goes, Tetris The Grand Master has a system that strikes a nice balance between unpredictability and usefulness, so you might find its algorithm useful.
Viewing post in Devlog: Puzzle Wizard - prototype for a puzzle RPG
Oh, thanks, this is interesting reading! I had a look around other Tetris systems on that wiki, and I also like the bag system here - that might be useful way down the line if I ever get this off the ground as a full puzzle RPG and want to put in status effects that give the player less of a piece.