You're definitely going to re-raise for a different nature (and at least a higher Speed IV, maybe not 31, but WAY more than 3). Since Lucario's a base 90 Speed Pokemon anyway, it's doubtful that it will outrun much, so you're better off with increasing it's Sp. Attack, and just Pass an Agility to it.
Lucario
Modest
@Wise Glasses
Inner Focus
252 Speed/252 Sp. Attack
-Dark Pulse/Shadow Ball (Dark and Ghost hit about the same types, but the only real difference is reducing sp. defense or causing a flinch, and you're probably better off with reducing sp. def. since Lucario probably won't outrun anything)
-Aura Sphere/Vacuum Wave (Personally, I'd go with Aura Sphere; Vacuum Wave is good for a priority attack, but that's all, and with an Agility Passed to it, you're biggest worry will only be Scarfers)
-Psychic/Metal Screech (If you go with Vacuum Wave, Metal Screech may be better since it'll let Vacuum Wave hit harder than normally).
-HP Ground/Ice (Ground hits Fire types while Ice hits Dragons, Grass, and Ground types, so you're probably be better off with HP Ice than Ground)