So I recently read a post where someone complained about why people who are actually trying their best constantly get matched with feeders and trolls, and I really understand where they are coming from. I know that a Prisoner's Island would make things worse, and I understand the reason why Riot are against it. Since you've recently taken inspiration from *Overwatch* for the new Honour's System, I have a suggestion...
There is a game called *Mobile Legends* which is literally a rip-off from LoL (but that's not the point). In this game, they have a ranked "health" bar, which maxes at 100. Every time you go AFK or int or troll or are toxic you lose a certain amount of that health. Once you go below 70, ranked becomes disabled until you get it back up above 70, which you can do by playing games with good behaviour (which give you 1 health, I believe). You don't need to win those games; just show you are no longer a toxic player. It also puts people who have had a high decrease for several games in a lower priority queue.
On the opposite end, anyone with higher than 90 or 95 at the end of a given week gets a reward (the equivalent to a Hextech Chest + Key). This gives players an incentive to have good behaviour.
If you can somehow integrate a system this like into LoL, it will prevent well behaved players who are trying their best from being matched with trolls and feeders.
Even if this can be merged with the new Honour's System (i.e. anyone with Honour Level 1 cannot played Ranked - since the default is Level 2), it would drastically improve game health.
Copy *Mobile Legends* health system, which decreases health whenever someone has bad behaviour, and below a certain health value Ranked becomes disabled for them, and perhaps even integrate this into the new Honour's System.