ZDoom

Lower/Upper walls are a major change and won't happen. As for 'changing height', no. A portal will always be the entire connecting area to the neighboring sector.
Keep in mind that the limitations here come mostly from keeping the constructs simple to check both in the renderer and in the game itself.

Would it be possible to add the ability to change the height of a line portal or link a portal to a lower/upper wall.

While I know using stacked sectors is a workaround for the most part, it can become a problem for buildings that are at different elevations which go beyond one stacked sector's height. It would be quicker and simpler to have the ability to set a height of a portal instead of working around it's infinite height.