People who can play all roles competently will be negatively impacted since being well rounded probably helped them climb to where they are currently. That's the boat I feel I'm in.
Being good at an unpopular role like Support is an advantage for you and I. When this new version of Team Builder goes live, we will no longer be occasionally facing last picks who have been forced into the role and don't have a clue how to play it. When we Support, we will now be playing other Support specialists every game.
Hopefully being able to specialize in less roles offsets that for you, but I think it might be a little naive to only see how the new system will positively impact you. Your opponents will benefit from the changes as well.
Since we know that it isn't being released this year, it'll be released during Preseason "at the earliest". If you avoid ranked during preseason, that means you have until January or February until you have to work with the new system. This means that you have, at least, 6 months to choose two "main" roles, and practice them. i figure that should be plenty of time to learn those roles. In addition, there are already ways to facilitate that: you can play Normal Teambuilder.
There's also other levers that Riot can use to make the system more palatable to "fillers":
1. MMR reset or placement matches may be different this season. i generally assume that most resets benefit people with "solo lane mains", but that's not 100% certain.
2. You may get a very slight MMR "handicap" or slightly boosted LP gains if you queue as "fill"
3. At the very least, if you queue as "fill" you'll experience much faster queue times. If you're able to keep up a similar win rate, faster queues -> more games -> more LP. Going 1-1 generally results in a net gain of 8 LP, right? Think of it as playing Face Hunter up to Rank 5...
4. In actual fact, "fill" isn't playing each role equally as often. Both you and pacov play mid much less than your other roles. Is that just because of Cow and me? Probably not. It also depends how many duo queues use the system. If lots of duos play, then you might see your "fill" mostly actually playing jungle/top.
5. Some people are going to queue Support with... non-support ideas. Just because they want the shorter queue time (i've got 75 minutes to play! Not enough time to queue as mid, but support i can play...). You probably won't be facing Lemonation EVERY game. Inb4 GDM.
