
Reduce base damage on abilities, increase total ad scaling on abilities, make his melee W and auto proc based on defensive stats, His negative gameplay comes from him being too strong with lethality, which creates a situation in which he basically becomes useless before 30 minutes or if behind.
I'd put a crit synergy on his Q but make it deal less damage naturally.
I'd make him stack tear faster somehow
i'd make his Knockback deal physical instead of magic damage to help with counter itemization, would do the same for his W
Give him some passive which allows him to use melee only effects in ranged and ranged only effects in melee BUT would have to have some special stipulations on this (Counter point not fully thought of)
I'd give him higher bonus AD per level and maybe a hair of attack speed but i'd keep him to be more of a defensive ranged champion than what he is now
Effectively jayce is currently played as an assassin and artillery mage, I'd turn him more into an adc and bruiser hybrid who utilizes Triforce, botrk, steraks, hydra, ruunans, mallet, death's dance, iceborn and manamune in his kit.