Nerf the most boring forms of damage:

1.) Lategame autoattacks are ludicrous, and equally ludicrously boring to get melted by at the end of a 30-minute game. This isn't only the case for marksman but for anyone that whose primary role it is to deal damage in general, even if they're AP. The only champions with justifiable amounts of scaling AD right now are tanks.

Lowering scaling AD would not take damage away from most abilities which make champions unique; rather nerfing autoattacks accentuates them, making their champions even more unique. Graves' positioning for his Q might really matter. Assassins might be more inclined to land all their abilities on their target rather than faceroll their keyboard as they right-click. Irelia wouldn't decimate everything so quickly with Triforce procs, nor Riven with her weaved autos. Four back-to-back crits from a fed marksman might no longer be enough to kill anything that isn't a tank.

2.) Similar to autoattacks lategame, nobody is wow-ed by mana ability spamming early-midgame. Nerf overall mana as well. That way the old mana items might see more play as nobody buys scaling finished mana items within their first 3 items anymore except the champion with the current lowest WR by a mile .

TL;DR: Nerf lategame autoattack damage and make early-mid mana ability spam actually cost something. These are the two most egregious forms of damage in the game.