This was the team I initially settled on. Magearna was my "Ol' Reliable" over the past few seasons, and in theory it should do the exact same thing as normal. Great typing, can take out Dragons without getting in harm's way, and bulky. The issue with Magearna is that everyone was either running Magearna, so tired of Magearna that they specifically brought Pokémon to fight Magearna, or both. Not fun.
When I went back to the drawing board, this is the team I settled on. The biggest issue with the team was that everyone was bringing a Fire-type to deal with Magearna, which meant that everyone had a hard counter to two-thirds of the team in the same Pokémon (Magearna and Lucario). I could have replaced both, but I figured I'd try replacing the Magearna first since that's the one people were immediately prepared for instead of taking the few seconds to process Lucario's typing to counter it.
Removing Magearna took out a piece of a very convenient Fairy/Dragon/Steel core. I pulled up Serebii's New Mega Evolution List and looked through everyone to find a replacement Fairy-type. Meganium was an option but was still weak to Fire, and Eternal Floette was an option that wasn't weak to Fire but had been meta relevant for a while (would people still have an instinctive answer to it?).
Then I remembered Base Clefable didn't have that Flying-typing, that was exclusive to its Mega form. Not to mention its Alpha Exclusive move was a Fire-type move, so if Clefable were about to attack a Dragon-type and I suspected they might hard swap into a Steel-type to counter, I could use a +Flame Thrower (hopefully with a Calm Mind boost too) instead. Or, if Garchomps were present and getting on my nerves, I could Mega Evolve and get a second immunity in the process that immmediately helps me.
Garchomp Z worked exactly as intended. Bigger Pokémon with huge destructive power. My wins and second place games were from over-relying on Garchomp Z's Earthquakes et al.. Definitely not a bad choice itself, and I would certainly consider using it again in the future. It does lose its Fairy-typing so its Aerial Ace lost its STAB bonus, but then it also wasn't as vulnerable to Ice anymore.
Finally, the Phantom Thief himself, Lucario Z. Lucario Z worked. Didn't really stand out much. Lucario Z's biggest issue was that it's pretty frail in both its Normal form and Mega Z form (regular Mega Lucario gained a bit of bulk, but was not an option this season). In a season where everyone has access to Mega Evolution, Lucario Z was a Pokémon that needed its Mega Evolution more than normal and still could have benefitted more.