I say that GTA style sandbox RPG would be great, but in first person. It would be hard, but would really get out into the game.
Think about it... morph bird, and see the world with the ultra detailed vision and see far away, morph a dog and have vision weird, but see like faintly colored scent trails to follow, morph an insect and see the compound eyes vision, morph a roach and see the screen shake when you're near human footsteps, have controls harder to work/become unresponsive when the animal's instincts are fighting you... It would be amazing.
Third person is okay for most games, but to really get a feel of what it's like to be an Animorph, you need to go first person.
I disagree that you'd have to "buy" morphs, I say that they be more like hidden packages or something, like you can get a golden eagle, so long as you find and catch one. The better the morph, the harder they are to get to.
I think there should also be a karma system with multiple endings. For instance, based on what choices you make, it can affect the ending. Do you kill human controllers or save them? Help the Hork Bajir Colony or not? Save a bunch of humans and miss your ONLY opportunity to get a grizzly morph, or ditch the humans and obtain the morph? Also, split decisions, moral conflicts in game... really get a feel for it, how it was like for the Animorphs in the book.
This could have the potential for a very long, very good game... but it will simply never happen. A big budget sandbox RPG like that is reserved games with a wide fanbase; it just wouldn't happen for a "child's" book series that's 10 years old. In order to really match the books, and to play first person, there would be some hardcore stuff. For example, if you were a wolf fighting a Hork-Bajir, you'd see your muzzle rip through flesh, and you'd see blood, missing limbs and bodies everywhere. For the intended age group of the books, the game would need an E or at the most T rating. It would be hard to pull unless you made the game really, really cheesy.
Also, to really make it work, you'd need a brand new engine that is very different from what current games run on. For it to be practical, if you get hurt, you would have to lose strength and mobility and vision, etc. based on the injury. This isn't just a red splash on the screen, find a medkit and reload, you would need to limp away bleeding your life away, and remorph. It isn't "as soon as you're not getting attacked, your safe." In the books, a big issue was demorphing before you lost too much blood. In order to really make it... limp differently based on how injured a limb is, loose partial vision when hit in the head, changes in controls for all animals that get damaged in any possible way, ALL the different motions and attacks and controls for dozens and dozens of animals... (It wouldn't be fun if you only could pick between a handful...) It's just too big of a job. It would be extremely expensive to do, and just isn't practical for who would buy it. Also, being a nothlit isn't game over, you'd just be trapped; if you find the cube, there would be countless of npc's to pick from with their routes; if you find a controller, you could trap them for three days to save them and affect the story, but how many controllers are there? If you become a controller, how could you be saved? There would need to be storylines for every possible thing you would do, every choice you make. This isn't standard RPG choice morals that lead to a max of five or six possible endings, you're talking about dozens. This isn't some "OKAY, you're a criminal. Kill people and get money and cars." This game would be more massive than anything previously made. It just can't be done today.
Assume I was a multibillionaire, and change everything above I said was impossible to "pay people to do it", and you would have the most amazing game to ever grace the surface of Earth.
If it was made half-ass, I would still play it, but be incredibly disappointed when I get five morphs and they have limited controls, and follow a linear path with no real choices, and no ability to attack what I want, defend what I want, or really do things with innocence and controllers.