Miraidon ex

play one ampharos ex it has massive health and you already have the evolution line

also if your going to play regiaeleki vmax look into playing the radiant eternetus

I was considering both of those, but I couldn't find room for the ampharos, and the radiant Greninja provides needed draw power/energy discard that can't be sacrificed. Miraidon can also find regieleki v very easily 

I personally have been play testing with a similar deck. Here are some changes I would recommend:

Cut Greninja for Radiant Eternatus (Also cut the Regieleki V's) 

Run Electric Generator from SV base set

Probably run a 3-3 line for flaafy and add an ampharos ex

Glaceon already explained why that isn't for this deck.

One thing I would majorly consider is adding Beach Court into the mix, due to the nature of Miraidon's attack, being able to just switch into another (Similar to Mew VMAX's free retreat after Cross Fusion for Techno Blast) is a major help, and although 1 energy retreat with Flaafy isn't that bad either, but maximizing the duration for use of the energy is never a bad idea. I also recommend maybe running just 2 or 3 Miraidon, just to get more utility out of more Trainers.

One thing I would majorly consider is adding Beach Court into the mix, due to the nature of Miraidon's attack, being able to just switch into another (Similar to Mew VMAX's free retreat after Cross Fusion for Techno Blast) is a major help, and although 1 energy retreat with Flaafy isn't that bad either, but maximizing the duration for use of the energy is never a bad idea. I also recommend maybe running just 2 or 3 Miraidon, just to get more utility out of more Trainers.

replace Judge (Fusion Strike) with Marnie (Sword & Shield) 

haalandagoat, Marnie has a "D" regulation mark, so it will be rotating when Scarlet/Violet base releases.