Earlier Seasons Jayce vs. Current Jayce
Remember when Jayce needed Muramana (haha what even is that item amirite? It's an Ohmwrecker 2.0 at this point). He scaled into the game and had comfortable powerspikes.
Now? Now he just builds lethality, and because this other stat comes with every item in the damn game, cdr. He doesn't have any fun or balanced powerspikes now, he just gets a single dirk as his spike and just scales from there throughout the rest of lane phase.
He went from a skilled champion to one that you either counter or you don't, and if you don't then go ahead and just start baking an "I'm sorry" cake for your teammates...it'll be a better use of your time.
Now if you've made it this far I will admit that he does indeed fall off a little bit late-game. He always did have the advantage of ranged as well, so it's not like old jayce was perfect, he was just better imo. All I'm simply saying that I feel as if his identity in the game has changed and it's a change that I simply do not agree with and one that I definitely don't enjoy.
Perhaps increase his mana costs to push him towards that Muramana again and in exchange give him something that'll scale into late. You could just increase his scalings but if you wanted to get funky give him an attspd/armour reduction effect on his E/W for his range/melee form respectively.
Idk a sure-fire solution. I know some of you wont agree with me but I've put my opinion on Jayce out there at least, and I'd love to hear you guys' reasoning for your opinions.