If Riot is serious about running this new role pick ranked system IMO I think...
they're gonna have to eventually implement some hidden scores on everyone's account. These scores track your recent role assignment. And you get more and more priority elsewhere if your scores aren't balanced with your selections.
If you pick a main and secondary role and you get secondary too many times it should give the system more preference to your #1 preferred pick. Maybe even bump you ahead in queue of others who have more "content" scores. Or simply just look for a secondary role game for you if your primary role score is above most people.
If you're a filler if role selection gets really imbalanced it should wait for the next best pick for you.
They should implement that the same time as incentives for fill players. That way # of fill players increase in general which also makes the whole system healthier and more efficient.
Otherwise Riot I think you really should just scrap this new ranked system entirely. There's absolutely no chance of queue times and matchmaking quality working acceptably otherwise.
It's possible it's already implemented like this though and it just doesn't work under the stresses of reality.
Generally I don't like the new system entirely tho. Guaranteed much longer queue times for most players across the board for a contrived system because a few people can't adapt to the game. Fill pick being non-functioning in average elo is a disgrace IMO.