We really need to add something to beat Ghostceus. I wholeheartedly agree. Dialga gives us something that can spread paralysis and phaze, making it easier for pokemon like Latios to clean late-game. However, we really need to add something like Darkrai to dispose of Ghostceus (Ho-Oh would work with RS support, but this is a Hyper Offensive team so I don't think a spinner is viable).
I am hearing a lot of talk about weather sweepers. Let's look at our current team and see how we stack up against them:
Life Orb Kabutops: A problem, but Dialga can take a Low Kick and kill with with Draco Meteor. Terrakion can revenge since most LO Kabutops forego Aqua Jet for Rapid Spin and run Adamant over Jolly (Terrakion outspeeds Adamant Kabutops).
Choice Band Kabutops: A major problem late game. Aqua Jet can OHKO Terrakion, Low Kick kills Dialga now after a little prior damage, and Kabutops murders both Latios and Deoxys-S with its STAB moves. Dialga can switch into Stone Edge or Waterfall and force it out/kill it, and Terrakion still outspeeds and revenges if it is stuck on a move other than Aqua Jet, but Banded tops can wreck this team with good prediction. We need to have an answer for this.
Life Orb Omastar: More manageable than Kabutops, but still a minor problem. If Terrakion locks itself into Stone Edge, Omastar will get a free Smash and wreck our team as is currently constituted. Without the smash, Dialga takes only ~55% or so from Earth Power and kills it. Latios takes 60-65% or so from Ice Beam and again kills it. There's not much you can do about Omastar as it comes in to revenge weakened mons, because if the opponent smashes as you switch out you lose. With that said, this is less threatening to this team than Kabutops.
White Herb/Focus Sash Omastar: Not a problem so long hazards are up.
Life Orb Kingdra: This murders Latios, Deoxys-S, and Terrakion while dealing decent damage to Dialga. I am really warming up to Dialga now as it deals fairly well with these weather sweepers! Still, if Dialga is weakened Kingdra will be a severe issue. Fortunately, Kingdra cannot hit nearly as hard as Omastar so whatever Arceus form we add (while likely incapable of dealing with Omastar or perhaps Kabutops) should be able to beat Kingdra 1-1.
Life Orb Ludiculo: The only weather sweeper capable of beating Dialga 1-1 as Focus Blast 2HKO's Dialga while Draco Meteor cannot OHKO Ludiculo in return. Latios easily takes an Ice Beam (does maximum of 50% or so) and wipes it out with Draco Meteor. Terrakion is outsped and beaten. Like Kingdra and Band Kabutops, Ludiculo is an issue we need to find a consistent answer for.
Chlorophyllers: Venusaur can sleep something but after the sleep Latios beats it easily. Victreebel is in the same boat. Shiftry is really scary thanks to STAB Sucker Punch and Swords Dance. It can also outrun and OHKO Terrakion. Nature Power (Earthquake) deals strong damage to Dialga. Shiftry destroys our entire team if either:
A. It gets up a Swords Dance (unlikely, but again, we must be careful about locking Terrakion into Stone Edge); or
B. If Dialga is weakened.
Things we can add to beat Kabutops include Giratina-A, Giratina-O, and Rayquaza.
Things we can add to survive a hit from a +2 Omastar that fit into a Hyper Offensive team include Palkia and Latias. That's it, but we have Omastar covered I think.
Things we can add to deal with Kingdra include Rayquaza, Arceus-Normal, Arceus-Steel, and perhaps a more obscure Arceus form. If we add both Rayquaza and an Arceus form, Kingdra will no longer be an issue.
Things we can add to deal with Ludiculo include Palkia, Zekrom (can take an Ice Beam), almost all Arceus forms, Rayquaza, and Latias.
Things we add to deal with Shiftry include Rayquaza (though Sucker Punch hurts quite badly, especially after SR...Rayquaza isn't a good check at all), Palkia, Arceus-Normal, Arceus-Ghost, and if we really want a counter, Arceus-Flying. Much like Kingdra, EKiller would mitigate our weakness to Shiftry.
Sorry to ramble, but I fear weather sweepers right now. I am guessing Darkrai will be added at some point to deal with Ghostceus -- as it probably should -- but that will only make us weaker to weather sweepers. Defensive Dialga was a solid addition, but cannot take on weather sweepers by itself. I suggest we consider Extremekiller, Darkrai, and Palkia as we continue to progress through this CCAT. Still, I feel we are off to a good start.