In normal games it's not necessary that the punishment is more severe, but I think for ranked you should instantly get demoted several divisons if you ragequit the game.
It's insanely easy to make an automated system. Player goes 0/x/0 and quits within the first 15 minutes -> obvious ragequit.
So if a player has these stats and left the game in that timeframe AND gets reported for being AFK/Leaving the game that player should get an immediate insanely harsh punishment. At the very least 2 leagues down and optionally a 3 day ban.
Repeat offenders should get increasing punishments that get much harsher much quicker than usual.
Nothing is more frustrating than losing a game because your idiot toplaner went AFK after 1 death, while your midlane is 9/0 and botlane is 0/0 with even CS.