The subcircuit symbol is very inflexible. What would be good is if it was made more like an 'element' graphic, allowing (in decending order of must-ness), via the parameter pane on the right, the user to:

1a: define the shape of the graphic (circle, rectangle, square)
1b: define where on the outline each port appears.
1c: resize and rotate the graphic (same way as elements, not via params)
1d: define the border width (incl off) and type (line, dotted, dashed)
1e: define optional fill colour

2a: select shape of each port (eg triangle in/out, square, circle, etc)
2b: define a symbolic name as optional alternative to file name

3a: specify a small image file to show instead of the graphic
3b: define the position of each port relative to the image origin
3c: rotate/resize the resulting graphic of 4a & 4b

.... Don't want much do I ? Though, really, I think 1a, 1b and 1c are really essential enhancements.

Doing 1,2 and 3, in a way, could be a great workaround for currently not being able to define custom components.

If a custom components feature were implemented, subcircuits would still be an essential feature so the enhancement of it would still be a good idea.