Slaking would likely require gyms to be completely overhauled. CP dictating placement in a gym is dumb and discourages teamwork and diversity. I would love for pokemon like Ampharos, Umbreon, and Steelix be able to shine on defense despite their below-average max CP.
Secondly, Slaking would need to attack twice as slowly on offense and defense similarly to Ditto. Meaning on defense, it would only attack once every three seconds.