I have always loved the concept of a shapeshifting. The idea of being able to morph into different creatures as the situation dictates is incredibly appealing to me. While it's been attempted in several games I feel it falls short in most of them mainly because it hasn't been thought through.
Common pitfalls (to avoid):
Forms don't scale
Baldur's gate 2 gives you a bear form at level 7 unless you have TOB you have that same form (same AC/attacks/damage) for the entire game. In general you either need level based bonus (like monks in most games), or forms that improve/get replaced as you go.
One useful form
You can be a spider, bear or wolf. They are similar except the bear hits harder and has more attacks per round. Why would I ever turn into anything else? If you're going to have different forms ask yourself why/when would the player use this form.
Nerfed forms
You can turn into a mindflayer but you can't stun, you can't drain intelligence and are pretty much a druid who looks like a mindflayer. I understand sometimes unlocking abilities over time, but if I can turn into a form let me use it's powers. Spiders can poison, basilisks can turn things to stone, nymphs can charm ect...
What's important to me:
Distinct Defensive Advantages
I want defensive resistances/abilities that matter. The ability to turn into a serpent and be immune to poison, or a fire elemental and walk on lava, or a creature that's skin is so tough arrows bounce off.
Abilities, Abilities, Abilities
I don't expect forms to be as good in melee as a well equipped berserker, but I assume that comes with the tradeoff I can do things a fighter can't. Can a werewolf howl and cause enemies to flee? Can a demon curse opponents? The ability to have unique and meaningful powers is a major part of what makes classes like this fun.
Appearance matters
I want forms that look cool and are interesting. Wolves are nice and all, but I want forms that are visually impressive. Creatures like: towering minotaurs, fiery elementals and trents.