Our current plans are not to allow a player to queue for only 1 position. A lot of players have suggested this idea and said they would prefer to wait a bit longer if it guarantees their 1 position; however, this design change would actually greatly affect the overall matchmaking system. Let's take 2 extremes for example. Let's say every single player chooses "Fill." In this scenario, matchmaking is fully optimized, and matches will start instantly assuming there are enough players in the pool to start a game, and they are close in MMR.
In the second scenario, let's say everyone chooses 1-Position. In this situation, queue times dramatically increase for everyone, and some players actually won't be able to find matches because our position balance isn't perfect. Right now, last we checked Support and ADC (surprise?) were highest in demand, and Mid was the lowest in demand.
But, let's say only about 10-20% of players choose 1 Position. This actually has a weird cascading effect. Those players will have higher queue times and wait longer in queue; however, the longer they wait in queue, the wider their MMR boundaries grow (which determines what teams they can join and match against), and they will typically end up on teams that are quite varied in skill. So, what happens is that a small design change could actually decrease the overall match quality of the entire system for all the other players and now everyone is affected by the change instead of just the players who wanted to wait a bit longer in queue to guarantee their position."
