Well, FF serie has a class named Morphist. They use the soul of monsters as weapon that allow them to temp transform into that monster to use their skills. Your idea is just a few adjustments/tweaks away from this class.

A good idea would be having an hybrid and full transformation system (No one deals with transformations better than breath of fire! So i guess it will be my reference =P).

As hybrid you can have your base skills and some of the monster's skills. As look you have an humanid version of that monster. As full transformation you fully transform in that monster, having no access to your skills anymore but having full access to the monster's skill. For balancing purposes, i'd add continuous MP cost while transformed (no MP = transform back).

IMO it would be a very interesting job..even in FFXIV...since you would need a certain sould to be able to transform into a certain monster and, since you cannot change weapon during fight, you will be restricted to only one monster type per fight, keeping things fair.