By any chance do you want something like in inkscape a rectangle with rounded corners?
If so get inkscape make an rectangle object to path export to simple svg and import(look for svg import builds)
Right off the top of my head, if I were to do this, the first approach I think I’d try would be to use animated “alpha over” shapes (one for each half of the rectangle) to gradually reveal a rectangle that was already there. The straight sections could be easily auto-tweened and then I’d probably frame-by-frame the curves.
That would be an interesting challenge. Synfig doesn’t have a native representation for rounded rectangles. Synfig’s rectangles all have square corners. I don’t know how (or if) the SVG importer would handle rounded rectangles.
To make a rounded rectangle in Synfig. I’d start with a bline circle, and stretch the tangents until they nearly meet at the corners. Animating the rectangle like you describe would be tricky. I’d be inclined to try just unmasking the finished rectangle using alpha over, or maybe applying a gradient to the line and animating the gradient, or animating the width of the line (may not be very easy to control how uniformly it comes in).
The object has to be transformed with object to path so it would become like regular paths that would be transformed into normal regions in the exporter
As genete points out, there is an example layer that does curved rectangles (it’s not distributed under windows by default). You’d still have to figure out how to do a slow reveal of it.
Here is a very quick and dirty approach to animate the gradual revealing of the rectangle. Hope I understood the question properly and, please note, I did not use a rounded rectangle here, but the approach would still work. The YouTube link shows the result and the Synfig file is attached below.