I'm a bit disappointed with how the Origin Formes of Dialga and Palkia handled stat reshuffling. (...) Diagla-O gets slightly better Defense, which is fine in a vacuum but doesn't really change how you use it. This also comes at the cost of Leftovers recovery.
Palkia wins out here since Palkia-O has 120 Base Speed. This is admittedly pretty awesome, but again not sure if the held item cost makes up for it.
Initial Thoughts: Oh, right, would make sense they would need to hold an item like Giratina would. So, did they do the smart thing and re-use the Timespace Orbs or did they...
OF COURSE they made new items! Why not at this point! And according to Bulbapedia the Timespace Orbs also still exist so you have essentially two of the same items just with slightly different names and effect. Which only makes both worse as, while Giratina's Griseous Orb not only changes it AND give its Dragon and Ghost Moves a power boost, the A-Crystal and L-Globe do not. And, as
Karxrida pointed out, that means the only reason you'd even want to change their forms if you want a Dialga who takes Special moves slightly better (aka completely pointless as it's SpD stat was already good and the mentioned missing Leftover recovery actually makes it less tanky) and a speedier Palkia (better of the two but once again missing Item slot means it misses out on a lot of potential). At the very least having the boosted STAB Moves would have meant there was SOMETHING the Item Slot was contributing that isn't a simple form change (and put it on par with Giratina which I thought was the POINT of giving them Origin Forms).
And I do agree just in general they're disappointing all around. Giratina's not only changes stats (and while it's just a number swap it's symbolic, going from a defensive form as it manifested in a dimension which doesn't match its physics to an offensive form as its using the energy from its home dimension which its the protector of) but also changes Abilities going from the usual Pressure (a mechanical representation of it using its deity presence to awestruck or frighten those attacking it) to the more specific Levitate (a skill which better suits its needs).
Dialga should have leaned into defense: 100/100/130/160/130/60//680 and Ability changed to Solid Rock.
And if we're going to make Palkia faster, let's make it fast: 100/100/90/150/90/150//680 and Ability changed to Limber.

That thing got Flash Fire.
And then it has Heatproof, which halves Fire- damage, and that halves burn damage. ig you can use it to take less damage from... Scorching Sands/Scald induced burn
I'm confused what you're trying to point out here. Remember, Rolycoly doesn't become Fire-type until it evolves into Carkol, so it's still susceptible to other Fire-types Burning it (also, if it was a Fire-type, it would be immune to Burn completely; and for this reason Carkol and Coalossal get Flame Body instead). Yes, normally no one would likely do that being it resist Fire, but there's always switching-in opportunities.
I was today years old when I learned Heatproof reduces burn damage
Well when the Ability is only learned by three Pokemon, each having other options which some people may prefer, it's not surprising that most people didn't know that little factoid (BTW I too just learned this today).
As much as I would also like it, the nuking if Megas and what happened to AshGreninja gives me no hopes :(
Yeah, unfortunately when there's no other "super forms" around the Pokemon which naturally get a form which is meant to compete with them sort of are left high and dry. If Megas return in a game that Necrozma (and maybe Eternatus) are in I wouldn't be surprised if they gave them a way to access their super forms again, otherwise the only Pokemon allowed to have a BST that's naturally (aka no held item (Hero Duo) or Ability gimmicks (Zygarde)) above 700 is Arceus.