... you know you have them! Those plants that you can't have (too big, wrong soil, wrong light, etc).
I love Gunnera manticata, but at full size it would take up about a quarter of my garden!
Same for Viburnum opulus (guelder rose). It would be perfect for this one spot in my front border... if it wasn't so big!
What would you have in your garden if there were no barriers to growing?