ACE 3! It's a canoncial sequel to ACE2. Excited? I have played 4 stages of it.

Warning: Spoilers below

UCE has dissolved since the first large-scale war against the Zentradi and many other enemy factions depicted in ACE 2. The war ended in peace, supposedly, but the Earth Federation secretly maintains a military task force known as Londo Bell.

Here are some of my observations...

Cpt. Amuro Ray

From: Mobile Suit Gundam: Char's Counterattack

I was wondering why Amuro was sort of "shafted" into ACE2. Amuro's appearance in ACE2 was merely a prelude. He is really one of the stars in ACE3, where Londo Bell formally appears, instead of just being mentioned (when I say 'formally' I mean Ra Cailum and Bright Noa's face appear on the screen as they are introduced to the player). I wonder how One-Year-War Amuro will be introduced into the game though. Now THAT'S interesting.
First appearance: Stage 1

EDIT: I've gone too long without seeing Bright Noah.
EDIT2: I finally saw him... but he spoke only a few lines (recorded in the production of ACE1, perhaps). First Gundam does not appear in the storyline. From some sources, I've heard that even Japanese players have not unlocked it, as it was extremely hidden.

The Dragonar Team - Kaine, Tupp and Light

From: Metal Armor Dragonar

These three fellas, along with Amuro, remain in the Federation after the war in ACE2 had concluded. Light appears speaking first. They have been quietened down A LOT. (They were really lively in ACE2)
First appearance: Stage 1 (Light Newman)

EDIT: Giganos and Meio Plat appear in the game, with Meio's motives unknown, assumed to be working with Char Aznable and Neo Zeon.

Ruri Hoshino and Nadesico B

From: Martian Successor Nadesico -The prince of darkness-

Yes, yes, they had to feature this. They had to "kill" Akito. But, I'm not disappointed yet. I like they way they introduced this series. Ruri and Nadesico are now part of Londo Bell.
First appearance: Stage 2

EDIT: Ryoko's Aestivalis is so much easier to use than Akito's Black Selena.

Isamu Alva Dyson and Guld Bowman

From: Macross Plus

Ah hah. This hot-and-cold duo has been in my mind for quite a few months now. (I even put up a picture of his YF-19 Excalibur in my workplace) Now they appear to be assigned to Londo Bell, deviating from their test schedule. (They are test pilots) Let's see how things will turn out. Look, they started to fight each other already (earning a Gravity Blast from Ruri, the way she explained her decision to them is hilarious). X-9 Ghost is mentioned surprisingly early in the game compared to in the Macross Plus OVA, where it was the concluding antagonist (sort of like a Final Boss).
First appearance: Stage 2

EDIT: They totally ripped out the background story to Sharon Apple's hacking. Well, I suppose it is pretty hard to tie the whole Macross canon together otherwise...

Garrod Ran, Tifa Adil, Vultures, Invaders

From: After War Gundam X (Garrod, Tife and Vultures), Shin Getter Robo Armageddon (for Invaders)

Here's the spoiler. Good work, Banpresto. They have re-introduced "interdimensional travel" into the ACE series. And that's a pretty logical way to put post-apocalyptic stories like Gundam X and Shin Getter Robo into ACE. I liked they way they purposely misrepresented the weird life-forms (original characters from the ACE timeline) as the Invaders in Shin Getter Robo. Mind-boggling but true. Won't you suspect them to be Invaders, when they were the ones who cause the apocalypse?
First appearance: Stage 4

EDIT: Those are confirmed to be real Invaders. What happened to the more monstrous-looking Invaders I saw in Super Robot Wars Destiny? Hmm

A.C.E. 3 Original

Here we come to the main character. He is a high-school student, unlike the UCE soldier you play as in ACE 2. He got hastily involved in the war (makikomareta). Cool! I think the storywriter intended to parody the stereotypical beginnings of late mecha anime like Gundam SEED, etc instead of going realistic like most of the other mecha games. Besides, this kid already knows what he is going to go through (he must have watched LOTS of mecha anime LOL). The Dragonar Team then convinced him that he was not the only one who has been this unlucky. (or lucky??) I have a good feeling about the A.C.E. 3 storyline.
First appearance: Stage 1 (duh, lol jk)

EDIT: The storyline is simply amazing. Though I think I've enjoyed A.C.E. 2 more. Probably because that was the first A.C.E. I've played? Or is A.C.E. 3 really rushed-out in terms of storyline and drama?

Series that do not appear

Eiji Asuka, Daba Myroad

From: Blue Comet SPT Layzner (Eiji), Heavy Metal L-Gaim (Daba)

They don't belong to Earth in the first place. I suppose Eiji had a "happily-ever-after" with Anna Stephanie and Daba Myroad returned with his gang to where they were supposed to be in their anime, far FAR away from Earth. (The L-Gaim anime did not have Earth in it)

Kou Uraki, Elphar Synapse, Nina Purpleton and Albion

From: Mobile Suit Gundam 0083: Stardust Memory

Stardust Memory. You start with these characters in ACE2. Kou Uraki and Anavel Gato were stars in the game. They are not in ACE3... Bernard must be somewhere getting himself drunk. (He's called Bernard right? I can't remember well, not even sure whether I spelt Synapse's name correctly... OH WAIT! Bernard didn't even appear in ACE2. Me and my imagination...)

Zeta Gundam and Double Zeta Gundam

Zeta Gundam appeared in the first ACE game and was never included in later ACE games. :((( I miss Kamille and Four.

Conclusion

Banpresto crossover games are a good way to get rigid Singaporeans like me to regain the ability to imagine. I absolutely love Banpresto for releasing ACE 3.

EDIT: Shucks, I'm getting interested in Overman King Gainer and Eureka Seven.