Special logos next to screen names for Tree completion
I think those users who complete trees should receive permanent markers next to their screen names to distinguish the achievement. Can you tell I finished today after starting last March?
The thing is, past a certain level of proficiency, practicising the same sentences over and over is no longer useful. If the stuff is already firmly lodged in your long term memory, better spend the time developing your skills futher in Immersion, or elsewhere.
The tree I keep gold is the one language that I barely know, where I need to practice constantly to get it into and keep it in my head. The languages I am fluent in, I don't bother: I do some refreshment as an idle pastime and to keep the sounds of the language in my head, but the decay algorithm is not well adapted if you have come to Duo with prior knowledge of a language.
So golden trees are a good measure of effort (but potentially misplaced effort) rather than necessarily of linguistic competence. That may not matter for the purposes of awards, colour coding etc, but let's be clear about it.