Starting with passive (open to new ideas) it should gradualy build up the speed and damage of the attacks per attack much like guinsoos rageblade scales to 25 stacks

the first spell should be bayonete charge where his attacks switch from ranged to melee for 10 seconds giveing him increased movement speed and the ability to walk through minions (ghost) aswell as to give him increased armour and magic resist for the duration of spell and say ten stacks of his passive

For the second ability (name open to changes) Musket barage he shoots a wall of damage much like ashes volley but instead of slowing it will cause a bleed over time the initial dmg will stack AD the bleed will stack AP

the third ability (again open to revision) crawl his movement speed is reduced but he gains a 100% crit chance for 5 seconds with slower attack speed but dodges all incoming attacks

For the fourth spell/ulty impale does a skillshot that scales with AP and does extra damage based on passive stacks which stuns the enemy for 4 seconds