Breloom is here to ensure that you are not swept by Excadrill who can wear down Scarf Landorus very quickly. It also helps out against Water-types like Slowbro, Azumarill and Suicune which can prove to be annoying for the team. Focus Punch allows you to punish Skarmory and Ferrothorn who tries to be funny and allows Diancie to get past them later.
Tangrowth @ Assault Vest
EVs: 248 HP / 56 Def / 204 SpD
- Giga Drain / Leaf Storm
- Knock Off
- Hidden Power [Fire]
Looking at the other 5 pokemon, I notice there is no solid Keldeo switch in, and even something as simple as Latios or Diancie can get really annoying since the Jirachi isn't fully spdef lefties and the Tornadus isn't AV. The other weakness is that grounds like scarf Landorus get really annoying when there are 2 big ground weaks and both of the current resists are fully offensive. Tangrowth just seems to be the best middle ground pivot vs all of the aforementioned threats, and it doesn't kill momentum that bad on an offensive team.
As for specific moves, Giga is kinda standard but Leaf Storm is also good to hit waters like Keld/Rotom/Mana super hard right away. Knock is standard for grasses/Skarm and whatever (this is really big for Keld + Dia). Lastly we have Earthquake + HP Fire to cover Heatran and Ferrothorn respectively.
As others have mentioned this team lacks a good Keldeo switch in, it also has problems dealing with Mega Lopunny without resorting to Protect shenanigans or Potentially losing Lando to an Ice Punch(/Beam), Celebi deals with all these issues, absorbs status for the team and can even pass a Nasty Plot to Keldeo or M-Diancie if you get the opportunity, alternatively yellow magic is always nice, Giga Drain is the best option as it provides recovery and hits Ttar that could be switching in on you decently hard, but Psychic can also be ran if you want to hit Mega Lopunny harder or deal with Amoonguss better.
The EV spread avoids the 2HKO from Specs Keldeo's Icy Wind after rocks, Mega Lopunny's Return after rocks and has enough speed for Adamant Bisharp (alternatively more speed can be run for Jolly Bisharp, the defensive EVs can be placed elsewhere if desired)
To keep up with the offensive presence of the rest of the team, Latios gives the team a solid Keldeo switch in. It forces a lot of Pokemon out and provides hazard removal as between Tornadus-T and Landorus-T you would be switching out a lot. Its really not much to say here as we all know what Latios does.
As already mention several times, this team has no Keldeo switch-in. Latias is a more consistent check to Keldeo than Latios due to her superior bulk. Latias also provides Defog support to get rid of hazards like Stealth Rock that limit Tornadus-T's recovery from Regenerator as well as wear down Diancie. Other options include but are not limited to: Psyshock over Psychic, Healing Wish over Recover (which I don't recommend as that hinders Latias' ability to check Keldeo), and Leftovers over Life Orb.
Tangrowth was taken, and every synergistic hazard remover was too, so I have to settle with a straight-up Keldeo counter, sand check, and physically defensive presence. Having a Water- and Electric-type stopper is really useful for this team, considering it's already pretty weak to hazards and fast-hitters like Thundurus get a free sweep on you if this team is weakened enough. Stun Spore lets you check Sand better, and HP Fire is good chip on things like Ferro and M-Scizor. If Thundurus is less of a problem than anticipated, then the standard Bold EV spread works too.