RAW are pretty straightforward here, and I would just tell the players: no. For example, as much an an amorphous placoid bear would be awesome, it is not what Wildshape does. You give up your species anatomical attributes when you wild shape, and since bears aren't anatomically plasmoids or changelings, they don't get to acquire plasmoid or changeling attributes from it.
To answer this question, you just have to ask yourself, "Could that beast normally do that anatomical thing? No? Then that's your answer." E.g. "Can a bear normally sprout tendrils? No? Then no."