How Judith's future career goes would have depended on her physical development and how her voice developed, as she was short for her age and was getting hormone injections to help her grow taller. She was extremely intelligent and loved voice acting and I have no doubt that barring some quirk of fate or sudden change in interests, she would have stuck with it. At the age of 10, I would've ranked her as the best young voice actress in the United States in 1988 (as the only three young voice actresses with more potential/talent at the time, Tara Charendoff (Strong), Cree Summer, and Alyson Court, all lived in Toronto).
As for Bluth, I could definitely see him casting Judith as the voice of Thumbelina rather than casting Jodi Benson as IOTL (though this would've probably been contingent on her being able to sound a bit more mature once she became a teenager). If he couldn't cast her as Thumbelina for whatever reason, possibly he might've had Anastasia made a bit younger in that film so that he could cast her in that.
Judith would've been a spectacularly good voice actress. If her voice had stayed mostly light and young sounding, I could easily see her as a more prolific Lacey Chabert type of voice actress, perhaps landing roles like Eliza Thornberry (The Wild Thornberries) or Gwen Stacy (Spectacular Spider-Man). If she develops some range, the sky's the limit, we could be talking about her in the same way we talk about Tara Strong or Grey Griffin (Delisle) today. Who knows, maybe Mary Kay Bergman takes her under her wing and it ends up saving her life too. I'm not sure she would've broken into live action because of her diminutive size, but who knows: maybe she gets cast as Galinda in Wicked instead of Kristin Chenoweth.
Such an amazing girl with so much potential, killed in a most horrific and tragic way.