Tetromino Slitherlink. Follow usual rules of Slitherlink. In addition, the gray squares can be partitioned into tetrominoes such that in each tetromino, each number from 0-3 appears. Pretty hard because it’s first time. Well it’s first time; I can’t estimate its difficulty properly.
Yay. Okay the tetrominoes are obvious, but putting the numbers aren’t that obvious.
I can obviously imagine a version where the tetrominoes can be ambiguous. As long as the numbers (and hence the loop) are fixed, it doesn’t matter how the tetrominoes are divided. Or otherwise, without given numbers. The latter would be way more difficult actually, because I couldn’t think of an opening without a number. Time to ask someone good at making difficult puzzles.