I've theory crafted a few in my day. I think my favorite was a Delusional King who thought he was king of Runeterra. He joins the League to show the might of his crown despite the fact everyone ignores him. His name would also be Gan so he would be "Gan King", word play on 'ganking'
Melee DPS, not tanky, assassin, mana champ
Passive (Hubris): Enemy champions that have less AD than Gan deal reduced damage (autoattacks).
Q (Leap of Faith): Skill shot blink (animation is a jump). Hitting stuns all enemies in range. Missing stuns Gan for the duration.
W (Reciprocity): Gan very briefly flashes his armor; any enemy debuffs aimed at him during that time will instead debuff the attacker. Long cool down. Successful Reciprocity reduces cooldown.
E (King's Hammer/King's Sword): toggle between using a hammer or sword for melee attacks. Hammer deals greatly increased AD but reduced AS and MS (from base). Sword has reduced AD but increased AS, crit chance, and MS. Low cool down. No cost
R (Long Live the King): While activated, all damage dealt to Gan is instantly removed one second after being received. 6 second duration. Killing an enemy champ while activated adds time to duration.