I'd favor a system that gave randomness, the character's "Transmuter" skill, and the availability of raw materials (creatures) all significant parts to play. For example, you'd have to start by capturing an existing creature, and because you're only a Dabbling Transmuter, you can only attempt to change a single attribute (probably size), and the change is in no way guaranteed to be the change that you wanted.
As your skill grows, you will be able to change more attributes at once, and more importantly, choose which attributes to change. But your control is limited: for each one, you can only choose Like or Dislike. "Like" can have effects like enlarging the chosen part, or enhancing it (bare skin might develop hair, scales, feathers, slime, etc.), or making that part more numerous (extra teeth), more numerous in a way that alters the body plan (extra limbs), or giving that part a quality that it didn't have before (a tail might become venomous, or prehensile, etc.). "Dislike" would have effects like shrinking the chosen part, simplifying it, removing things like claws/digits/limb sections, or removing the part entirely.
As your Transmuter skill continues to improve, you will develop the ability to graft parts (and/or qualities) from one creature onto another. Again, when you're just starting out, this will be very hit-or-miss. It will also progress in stages: At first, you will need to physically remove the parts from the source critter & (attempt to) attach them to the target. Later, you will only need to have the captured (or dead) source animal in your possession, and then only parts (bones, hide, etc.) from that creature, and then you can move on to animals that you have merely seen, and finally, you need only have read about some creature, in order to call up its description and Like/Dislike any aspects of its form that you wish to transfer to whatever abomination you're creating.
And finally, at skill levels of Master Transmuter & above, you can start to graft the qualities & properties of inorganic substances onto your creations. If you have made, e.g., a gigantic humanoid, you can replace all its tissues with some material like, say, bronze, and send it out to terrorize random settlements.