A larger bench in general could help. It wouldn't be a solution, but it would help.
But I disagree with increasing it to 3 per role.
Using a 6-man Fantasy league and an arbitrary single role as an example. Expanding to 3 per role wouldn't necessarily help, and would expose the system to abuse.
Currently, a 6-man Fantasy league allows 12 ADCs to be rostered (in either starting line-up or bench positions).
Most weeks, only 2 EU teams will be playing 2 best-of-3 matches. That's 12 viable ADCs per week (10 from NA and 2 from EU), regardless of tier.
With being able to roster 3 per role, it is possible for 4 players to monopolise all viable ADCs, leaving the remaining 2 players with a choice of 8 guaranteed sub-optimal picks.
This would be worse in an 8-man Fantasy league. In fact, even if both NA and EU played 2 best-of-3 matches per week, 7 players could starve 1 player from having any ADC picks at all.