Don't fool yourself, it's always been a marksmen meta. (But they will actually be fun in 5.22)

We called it a tank meta when saw tanks in the jungle or top half the games, or assasins meta when they were half the mid lanes, all while you've been able to count on two marksmen bot lane the vast majority of games since S1, when you'd count on one mid.

ADC's serve two irreplaceable purposes - to force enemies to build armor, as HP alone is not enough to tank ADC's but is usually enough against bruisers and AD assassins, and to take defended towers safely. Neither of these are particularly satisfying elements of play for an ADC, but this has historically ended up being their primary point of balance.

ADC's are about to get real differences in play style and itemization, while maintaining the above points, and with variety comes viability. Now that they can do more things and cover their weakness better with itemization, they will crowd out many mids, and some tops. Expect atleast 4 ADC's a game.

I'm not sure if this is entirely a bad thing, but this meta will blow all others away by scale. The reworked marksmen all look REALLY fun to play, and even with them crowding out certain other picks it will be an interesting preseason!