Jayce, the Defender of Tomorrow
Though his transformation gameplay hook is novel (or at least was, at the time), Jayce has two main hitches: the first being that he falls into the dual-kit trap of "he just wants to blow all 6 skills in quick succession" and the second being that, despite his role as the Defender of Tomorrow, he's actually much more of a bully.
That aside, the throughline for either half of his kit is fuzzy at best. In Cannon mode, he does general long-range stuff, and in Hammer mode, he does general melee stuff, but unlike Gnar (the poster-yordle for good transformation champs), neither mode has much of an identity.
The solution of the first issue provides an opportunity to further flesh out Jayce's mechanics (heh) - or more specifically, those of his weapon. By defining how it functions with a handy Hextech Capacitator (and distinguishing between its modes' functions), players will have to more carefully consider how they use their abilities.
With this newfangled version of Jayce's signature weapon, the next matter is how he uses it. This iteration of Jayce, above all else, should be a character that more fragile teammates can feel safe near. Defending should now come more naturally to him, with his Cannon's ability to dissuade aggressive tanks, and his Hammer's ability to shut down dive-happy assassins.
While few of Jayce's original abilities remain verbatim (with his Cannon combo going to Janna), I've made sure to keep most of his main functions intact - such as the Cannon's rapid-fire and Hammer's knockback - as well as keeping the same overall amount of mobility.
Also, with Poppy's redesign currently pending (but leaning into the direction of a supportive squire with a legendary weapon, following her revamped narrative), I've slipped two of her core abilities into Hammer mode's functionality, albeit different enough to not steal her thunder should she end up keeping them.
All values in blue are meant to give a general idea of the ability's properties, and are subject to change.
Recommended Positions: Solo Sidelane, Solo Midlane
Innate: Hextech Capacitor
Jayce's weapon generates a Gigahex Charge every X seconds after not attacking or using abilities for X seconds, and can store up to X Charges.
While his weapon is in Cannon Mode, gaining a Charge grants Jayce X% bonus movement speed for X seconds. While his weapon is in Hammer Mode, gaining a Charge shields Jayce for (X% max HP), up to (X% max HP).
Q: Shock Blast/Thundering Blow
CANNON: Jayce fires a pulse in the target direction that attaches to the first enemy struck for X seconds, dealing X/X/X/X/X + (X% total AD) physical damage once it expires. Damaging the enemy it's attached to reduces the pulse's remaining duration by X seconds.
Shock Blast will expend 2 Gigahex Charges to galvanize the pulse, stunning the target for X seconds once it expires.
HAMMER: Jayce slams the ground with his hammer, creating a shockwave in the target direction that deals X/X/X/X/X + (X% total AD) physical damage to enemies within it.
Thundering Blow will expend 2 Gigahex Charges to galvanize the shockwave, creating an explosion after X seconds that deals X% of Thundering Blow's initial damage and grounds enemies for X seconds.
W: Hyper Charge/Power Charge
CANNON: Jayce powers up his cannon, generating a Gigahex Charge every second for up to X seconds. If Jayce has at least 4 Charges, Hyper Charge can be re-activated within X seconds to fire a barrage of X + (X per Charge expended) shots at the target enemy over X seconds, dealing X/X/X/X/X% + (X% per X AD) of the target's health as physical damage per shot. If Hyper Charge is not re-activated, Jayce will generate 2 extra Charges.
HAMMER: Jayce powers up his hammer, generating a Gigahex Charge every second for up to X seconds. If Jayce has at least 4 charges, Power Charge can be re-activated within X seconds to slam the target enemy with a powerful swing, dealing X/X/X/X/X + (X% total AD) physical damage before knocking them X units backwards and rooting them for X seconds. If Power Charge is not re-activated, Jayce will generate 2 extra Charges.
E: Shock Boost/Lightning Field
CANNON: Jayce vents a magnetic burst from his cannon in the target direction, launching himself X units in the opposite direction if any enemies are caught within it.
Shock Boost will expend 1 Gigahex Charge to overclock his cannon, reducing the armor and magic resist of enemies hit by the burst by X/X/X/X/X% for X seconds, and launching Jayce even if no enemies are caught within it.
HAMMER: Jayce plants his hammer into the ground and creates a field in an arc facing the target direction that becomes a barrier after X seconds, which knocks down enemies that attempt to dash through it and disappears after X seconds.
Lightning Field will expend 1 Gigahex Charge to overclock the hammer, creating the barrier instantly and causing it to stun the first enemy to cross it for X/X/X/X/X seconds.
R: Mercury Shift
CANNON: Jayce switches his weapon to Hammer Mode, gaining a melee attack. While in Hammer Mode, Jayce sends a jolt through nearby enemies when they damage an ally champion, dealing X/X/X/X + (X% total AD) physical damage (X second cooldown).
HAMMER: Jayce switches his weapon to Cannon Mode, gaining a ranged attack. While in Cannon Mode, Jayce's second consecutive attack against a target deals X/X/X/X + (X% total AD) bonus physical damage.