I'll make a summary:
Wario vs MK
First thing to understand: Playing aggressive vs Wario is a recipe for failure. We have well documented cases of MK players thinking they can beat Wario in his face with speed, and then they get shield grabbed 50 times in a row and die. As boring as you may think it is, the best strategy vs Wario is to play the waiting game. Your goal should be to use your range to force Wario to approach as Unsafely as he can. He only has two choices, he can either take a risk and approach you, or he can wait for another opening. If he is down % wise, he's eventually gonna have to approach you, and under the stress he might make bigger than expected mistakes for you to capitalize on.
If Wario gets the lead and wants to play a keep away game, don't bother too much chasing him in the air. MK's strong in the air, but he moves slooooooow compared to Wario. Try to focus on where Wario will most likely land, and try to strike him with something relatively safe.
Wario can't compete much on the ground with MK, just using Ftilt, Dtilt, and Fsmash to zone him. Knowing this, he knows his best option to approach is to come in from the air, and then land close to you to get a grab/Fsmash off. Occasionally he will Dair you, but it's fairly unsafe compared to airdodging into MK.
First match, take him to either FD or Smashville. Battlefield is ok, but I think it might be the worst neutral for MK when it comes to facing Wario.
As for counter picks, pick something that is a safe bet, like Delfino, Possibly Frigate, (if you play gay enough, Brinstar is for you too), and Rainbow Cruise is a MAYBE due to the fact Wario can also abuse that stage pretty bad.