How to Buff AD Assassins without actually Buffing AD Assassins
Lower the base armor of squishy champions, so AD assassins can still do their job against them without us giving assassins some form of extra damage or bonus penetration so that it fucks with bruisers/tanks. I honestly don't care about ADC's being able to do sustained damage and lifesteal for days, and AP assassins will still be able to burst down an ADC so no need to deal with any sort of major MR nerfs. The AD assassins could use a bit of help, but they don't need to be able to just damage everyone.
So the meta returns to - in theory - what it should be.
ADC's will clean up the teamfights and proceed to siege objectives.
Mages will poke and burst down enemy champions and set up the initiation for the engage.
Assassins will try to take out high-priority targets who are squishy.
Tanks will try to pounce on an initiation and try to peel and cc down the team.
Bruisers will rampage, trying to take focus in a teamfight and do as much damage as they can in the meantime.
Supports will try to exercise their utility and turn the teamfight around for their team.
This is how it should be. Every member of the description should be able to perform to the expected level, and while the meta may shift to favor one role, in the end balance will always be achieved.