Since tweakeroo already combines and improves upon all my favorite mods, I figured I'd suggest another handy utility mod that would be amazing if added: a 3D measuring tape (i.e. select two blocks and it outlines the distances between them).

Examples (neither of which work for 1.15 but they have screenshots to show the idea):
https://www.curseforge.com/minecraft/mc-mods/block-meter
https://www.curseforge.com/minecraft/mc-mods/measuring-compass

This will most likely be added to MiniHUD instead, as that mod is generally for overlays and information. It will probably be a new shape type in the Shape Renderer system.

There were already column and box shapes planned for that system, among other things, so I guess adding the distance displays and making the shapes controllable by a "tool item" might even fit to those shapes instead of necessarily adding new shape types for measurements... but I can't say for sure before starting to implement it which solution feels the best.