I had actually been toying with this thought for a while and here's what I came up with.
It was mentioned earlier that it was Carbuncle's plan to create pacts between the five races and the Celestials to prevent their rejoining of the mothercrystals. However, now that Karaha-Buraha and the Champions of Abyssea (read: Vana'dielian Adventurers) are gone and their Summoners with them, the Celestials of Abyssea likely went on a field day.
Pacts are apparently a strong thing with Avatars of all kinds. If not for Nag'molada keeping the pact between the Kuluu and Bahamut (as seen during the CoP 2-5), Bahamut would have destroyed Riverne after appeared and summong Vrtra, Tiamat, and company. Fenrir likely wouldn't have saved Windurst during the Great War either, if not for Karaha and his strong ties to the current Sibyl.
However, the Celestials in Abyssea are likely just running amok for the sake of it. Ever have that feeling where you just wanna lie in bed all day, do nothing, play some FFXI? The Celestials are asleep for millenia and you want them to get up and work? Pffffft. Shinryu is already extradorinarly close to executing their plan for them, so what do they have to do? They might help him along by attempting to destroy the resistance a bit, but the end is neigh, in their eyes, so why not have some fun before Paradise is returned? I mean, it's not like they can go on killing sprees when there's nothing to kill.
Also, remember, not all Celestials are roamers in Abyssea areas. Some are stuck between Abyssea and Vana'dial as Voidwalkers. Which leads me to wonder what happened to Abyssea's Dynamis if Diabolos was similarly corrupt and is a Voidwalker. I have a bad feeling we may see Fenrir, as he is the only one we don't have a monster of/theory for. Bahamut, as someone said, was likely destroyed by Shinryu, Carbuncle said 'Eff it.' The way I see it, actually, Adventurers shouldn't be able to summon in Abyssea for above reasons, unless Avatars can transend dimentional barriers as well. I suppose you could argue that, since Abyssean Avatars can Voidwalk, Vana'dielian Avatars can transcend dimensions. But then that brings up an interesting point: Why is it only the Adventurers of Vana'dial are the only ones that care about Abyssea? You'd think that the Terrestrials would know about it and would want to do something. *shrug*
What I would like to know is what happened to Abyssean Kuluu and why they're even more corrupt as well. In addition to that, and maybe I just haven't done enough quests within Abyssea (realistically, I've done like, four, aside from mega boss quests >.< ), I'd like to know what happened to the Abyssean Beastmen. Surely they met the same fate as the five races, but their story is largely (read: completely) left out. And what of the Mithran Homeland (why can no one remember what it's called? thought it started with a 'G'...), Aht Urghan, and the Far East? Am I leaving out a continent to the west and/or the homeland of the Orcs/Gigas to the north? MOAR CAVERNOUS MAWS PLS.
As I'm typing this, I'm answering my own question, but left it up for conversation sake. The Tonberries blindly follow Promaithia and have likely recognized his advent via Shinryu and aligned themselves with him. Sucks for them that they get wiped out when he finishes reforming the mothercrystals.
Also, I understand the need for the mechanic of Relic armor in place in Dynamis, but since it's clearly stated many times the Empire stayed out of the war, why is it the armor of the elite Immortals (you know, the ones that don't go anywhere or do anything without command? likely, this Immortal would have been GIVING the orders), the best court entertainers (c'mon, us PUPs don't much like war...) , and Ephramaudian descendents (aren't they too busy leading a rebellion?) show up in Dynamis areas? I can understand some deserters/travellers...but it's HIGHLY unlikely that the 'best of the best of the best, SIR' decided to travel to Xarcabard and die. I fully understand the reason DNC and SCH gear is there and why it just decided to show up after WotG took place.
Thoughts? Ideas?