It's already great mixed attacker in sun team, I'm using it in my doubles sun team, Leafstorm ohkoes Azumarill and Knock off ohkoes Latios, Chandelure etc. Fake out to support team and Sucker Punch in case of emergency and to be able to do damage before getting finished by bullet punch/mach punch.
I really like Shiftry as a sun team mixed attacker, if it were to get a mega, I'd like to see it get +30 in both attacks, 10 in both defenses and 20 in speed and keep the ability Chlorophyl, its great ability but just requires the sun support. Another option would be more speed and offence and give it Protean.