Cube Waypoints is a waypoint mod that highlights marked blocks instead of placing a floating icon in the world. It's client-side only and designed with Hoplite-style PvP and trapping in mind as opposed to regular play.
To mark a block, look at it and press the keybind.
The mod has keybinds for two different colours of markers without changing settings, but an unlimited number can exist at once. Waypoints currently don't persist between closing and reopening the game.
Cube Waypoints is fully open-source, meaning it's free to use and modify with attribution.