Edited on laptop speaker so mixing probably not great.

Try to keep in mind that if a part feels full already, you probably don't need to add anything to that part. You can change parts of whats there but probably adding something new like a melody, when there already is a melody, makes things hard to focus on and sounds cluttered. If there exists a melody but you want to add a different melody, delete the existing one first so they don't compete.

The easiest and most productive thing you could do with this is probably to add a new section all together after the end, and since that will be quite empty there will be a lot you can add to it.

There aren't a lot of transitional elements in the song currently so adding a riser or buildup or something at certain places may augment the song. Just avoid making them too loud,you don't really want them to overpower the melodic elements of the song