There’s a somewhat common variant for shading puzzles where exactly one cell adjacent to each border segment must be shaded. I’d never seen it applied to loops so I thought I’d try and make a new loop genre from it. As it turns out, this genre already exists under the name of Wall Maze, though it’s fairly obscure. Here is an introductory puzzle.
Rules: Draw a non-intersecting loop through the centres of some cells. If two cells are separated by a border, the loop visits exactly one of them.