It seems that every game i have played on the pbe i have either had someone go afk, be toxic, or both. It seams that because players can just go back to playing on the live server they never seem to care about the game being played on the pbe. They are extremely quick to write them game off as a wash, and then proceed to go afk or be super toxic, if not both.
I would suggest that the leave buster on the pbe should be tightened up to a mere 2 games before a ban is ban placed on both the live and pbe account for the player. I would also suggest a more heavy handed approach to filtering toxic people on the pbe. Obviously the problem with people being toxic will never go away until deep learning ai is good enough to immediately decide if someone is toxic or not (faster and better than any human ever could be), but the leave buster could easily be tweaked to be more heavy handed.
Also, after you go afk in a game, it could be made to put people on a one or two hour game cooldown even if no previous offence happened. that way if someone legitimately had to leave, they arnt on the brunt end of the stick. And the people who went afk just because they didnt want to play would feel the punishment.