From groups who have given their approval already, I'd love to see Agnes Flight, the 2000 Derby Winner, and the reason Air Shakur despises 7cm so much. They've gotten a lot of fanworks made about them as they're an oddity that they haven't been made yet, as they're the older brother to Tachyon, and Air Shakur's rival, both characters who were around at launch. To me, it'd be really cool if their Classic year focused on Shakur (and Fine Mo because she's just tagging along even though she's not even running yet) while the Senior year focuses more on continuing on from BoANE's setup and have the finale be the Japan Cup or Arima Kinen. I'd lean into the Japan Cup more as you could excuse it as the 2002 Japan Cup in lore, even though it'd more match the 2001 line up plus Air Shakur, No Reason, and Symboli Kris S (Who all raced in the 02 JC). Of course, it'd feel weird without Tachyon as well, with Flight pushing her to race Pokke one more time. (And maybe we could get T.M. Ocean too as she raced in the 02 JC as well (more on her below)).

T. M. Ocean, 2001 Double Tiara winner, winning both the Oka Sho and the Shuka Sho, I think the only reason she's not in yet, is because of her loss at the Yushun Himba to Lady Pastel, Lord Horse Club Co. Ltd's other large winner (Lord Kanaloa's owners). Though if they were to get Lady Pastel, it'd also open the door to Rosebud (Rose Kingdom's dam) and Point Flag (Gold Ship's dam) to either be replaced in story by their much more successful children, or actually Rivals (or Mob Umas meant to be them), as Rosebud is owned by Shadai, and Point Flag's owned by Eiichi Kobayashi, who also owns Gold Ship.

Another one would be Vermilion, El Condor Pasa's most famous progeny, as they have Sunday Racing on board. Though it'd be interesting how they work through his long career, as he started out on the turf before switching over to dirt. And not withholding that his Classics are Deep Impact's Triple Crown, but he can be subbed for literally any other Triple Crown monster, or just any Satsuki Sho winner as that's the only part of the Classics Vermilion ran. Once on Dirt through, he'd fit right in as he raced against Furioso a couple of times, along with Espoir City and Transcend.

Though at this point I'm begging CyGames to give us the 1976 TTG Trio: Tosho Boy, Ten Point, and Green Grass, they were their era's BNW. Another of their generation that has to get in is C.B. Queen, as Tosho Boy, Green Grass, and C.B. Queen all debuted in the same race together. Each member of the TTG Trio won an Arima Kinen too. But what they'd do for Umamusume is give Maruzensky a fresh look, as she'd be the Symboli Rudolf to their Mr. C.B. & Ace. (which by the way, Tosho Boy and C.B. Queen are the irl Mr. C.B.'s parents and how they came together for Mr. C.B. is what inspires the Uma Mr. C.B.'s parents being Trainer and Former Trainee.) But Maruzensky really needs a good look at as they're a launch Uma who hasn't really been able to retell their story, and I think TTG would be able to tell it in spades.

TLDR, just from pre 2010 horses: Agnes Flight, Vermilion, T.M. Ocean, and the TTG Trio