[twaijdoftc]

I believe Xournal++ will be what you are looking for (once it is out of beta).

While not open source, there's also Styluslab's Write which saves the files as svg so you can open them anywhere.

[neonnoodle]

Another vote here for Stylus Labs Write, you can make your own SVG templates and re-usable elements. The only limitation with it is that you can't enter text.

[not_knuth]

Wow, Xournal++ looks exciting!