Yeah Flightsim.to seems to become the biggest freeware player… And 50 smaller download places exist… Last month I found a Spanish language website with liveries for various South American and pacific airliners. There’s Dutch websites with sceneries and KLM-liveries… Dream on, I think it would be very difficult on the long term, to monitor and review (!) all possible external liveries. But for MP gameplay it is essential it works. That is not a bug, nor a feature YET… it is a challenge. On social media, I can upload an avatar picture. In MP flight, it should be just as easy. Maybe only the tail image with your personal picture ? It should be possible to use liveries you create yourself, without the need for distribution. And MP players still see it 
that would be great.