Exte is a path-extension tool for After Effects from Terrible Junk Show. It makes extending a path easy: select two points on a path and a new shape layer containing the extended path is created, producing an extended line from a shape with an animation.
Beyond the basic extension, the extended path can be converted to a bendable form. You can also batch manipulate path points by selecting multiple points and controlling them directly with a single null, handling several points at once instead of one at a time.
A trial version is available, limited to creating one extended path. Documentation is provided in both an English version and a Japanese version, and a demo video is available. Exte supports After Effects 2025, 2024, 2023, 2022, 2021 and 2020, and is listed here as free.