Under current methods of operation adding levels to a game for distribution to users would have to go through the Developer and the update process at Google Play or other such site. Under this scenario the Developer would need to receive, test, check for duplicates, reject or approve and add to a future update. I suspect this would be very time consuming unless some how automated.
Enabling users to create new levels on their own device is another possibility. Obviously distribution would be limited.
Letting a user post a "level file" for anyone to download and use in the game would be fine if it was something like a maze game. If it was a word related game, and the "level file" contained word or phrases for text type of puzzle I can see a lot of bad possibilities, for a game with your name on it.
If it's a free game that turns out to be extremely successful with moderate advertising revenue and you realize people would actually pay money for it, is that a transition that is even possible in a level contributor format?
With the number of games coming out and developers stretching there imaginations to find game variations I am sure this type of format will exist and a methodology will be developed to ensure quality for those who care. If Pocketreers attempt had been more successful I am sure he would have many imitators and there may very well be an opportunity for that game to be resurrected.