In the GW2 context, though, there's really two main ways in which you can achieve transformations: You can do it through the profession special mechanic, or through utility skills. This means that, mechanically, elementalists and engineers are already acting as shapeshifters - elementalists have the 'change your action bar through the special mechanic' bit, while engineers do the same through utility skills. One could imagine, for instance, reflavouring many engineer kits as tranformations. A transformation profession, then, would thus be likely to end up having a similar playstyle to one of those.
With a relatively limited list and if kept to humanoids, I could see it being done elementalist-style, as there's more to the elementalist style than changing attunements - a shapeshifter would presumably be less focused on support and area effects and more on melee or single-target ranged, for instance. However, this does raise the question of where weapons fit in, and unless the answer to that question is 'it determines what forms you have available' (which would be even more of a mechanics-over-lore than skills being set by weapon already represents - if you pay attention to both friendly and enemy NPCs, you'll see them using skills that aren't available with the weapons they're wielding) then you'd be limited to four or five forms, including your natural one.
A more elegant solution would be through utility slot skills, as this would allow for a wider range of possible transformations. However, this would likely end up with a playstyle like the engineer - a shapeshifter would, again, presumably be more melee-focused than the engineer is, but the engineer already has kits that make them at least melee-curious, and mechanistically anything a shapeshifter could do, you could probably come up with a new kit or weapon for the engineer that would allow them to achieve a similar result. In fact, when you get down to it, Elixirs S and X already allow transformations of a kind, and transformation potions in general are a common trope - one could imagine engineers developing a line of transformation elixirs that allow them to be the transformation profession directly.
The big issue here is that it's a lot easier to come up with a new concept then it is to come up with a new playstyle and set of mechanics for it. Here, for instance, elementalist, engineer, and shapeshifter are all fighting over the same general playstyle - that of a character who has a wide range of options at their disposal and can adjust on the fly to whatever seems appropriate for the situation. Since ArenaNet found two mechanics to cover this and provided the two Es with different flavours of supporting skills, they managed to get two distinct professions out of it... although engineer and elementalist are still pretty close to the boundary of how close two professions can be while retaining their distinctiveness once you look past the magic/technology difference. There isn't a simple third mechanic I can see for the shapeshifter to use instead of stealing the mechanic of one of the existing professions, and the end result seems likely to end up feeling like an engineer with different graphics and more of a melee focus.
What might work is putting the shapeshifts on the utility slots and have them change up your entire bar and last for a set amount of time before going into recharge - kind of like mini-elites, except the profession is able to have more than one loaded at the same time. This would create a style that's less focussed around switching smoothly between options, but having something you can jump into for a short burst of power or a quick dip into another role, then back to normal play. This probably wouldn't be enough to form the basis of a profession - we'd still need to establish what the profession would do when not transformed - but it would be a way to distinguish between elementalists (profession mechanic attunement switching) and engineers (which can switch between kits whenever they feel like it.
Many good points! I like your elegant solution to making morphing utility skills. This leaves our F skills open to more class based options.
I would say a possible solution is to make some of the morphable creatures more melee based than either the ele or the engi. In this way we have a third possible "soldier" class but with completely new tactics. In regular form or possible humanoid form give them a new weapon a whip as a possible new weapon with slightly longer range but not as ranged as guns or bows or magic, but longer than typical melee--sort of mid field aoe range.
You are right in that their is an apparent scarcity to how many different mechanics are available to implement without coming to close to other professions. However, their is no scarcity of the different creatures that could be morphable using mechanics througout all of the already available classes. By creating this class as having different creatures that are all a bit good at all with limited ammount of morphability or extended cooldowns we can start to see a whole new battle strategy.
It has to be where going into battle the player has to be somewhat strategical about what morph he chooses--somewhat like engi or ele but our morph perhaps has different cooldowns or lasting effects.