There are many options that can go in the last slot:
Superpower: In my opinion the best move that you can fit in. It gives a lot of neutral and super effective coverage, while also hitting Pokemon like Ferrothorn and Ice Pokemon very hard.
Stone Edge / Rock Slide: Move depends on whether you want accuracy or power. It also gives pretty good coverage and a good way to hit Flying types like Talonflame.
U-turn: Gives good switch initiative, and is well complimented by Choice Band and Scarf sets.
Fire Punch: For coverage this is probably the best move you might get. It works best on Life Orb and Banded sets.
Other options include Roost and Defog, which are becoming increasingly common as Bulky Flygon was devised.
As of such, pretty much all of the above moves outclass Steel Wing (Fairy coverage is not always needed; most won't faint to it either with 70 BP), so it's recommendable to replace it for the aforementioned moves.
Hope I helped!
