Question: what do you do after you beat maylene pokemon platinum?
Answer: 30 Lucario. After defeating Maylene, you'll receive the Cobble Badge, which allows you to use Fly (though you haven't gotten this HM yet) outside of battle. Also, she'll give you TM60 - Drain Punch. Outside the gym, Dawn asks you to help her get her PokeDex back from Team Galactic.