There are additional problems with Nakoruru (which are actually concerns that are typical of most SamSho characters, but to greater are lesser extends): her visual style does not work well with Soulcalibur's approach to modelling, its graphical fidelity, or its art design. Most people who I have seen propose this or that guest character over the years do not seem to be thinking much beyond, "Ooooo, I like that character it would totes awesome to see them in this other game, fightin' 'n stuff!" They don't spare much thought for the feasibility or difficulty of translating that character into an entirely different game with its own technical considerations in a way that can be both a) faithful to the original design of the character (without looking like garbage) and b) works within the mechanics and limitations of the modeller and the environment.
With a characters like Nakoruru and Earthquake, who have a more cartoonish aesthetic and/or peculiar proportions, that already difficult task becomes still harder. For example, can you imagine how much work would be involved just in working out how to make Nakoruru's hawk work in three dimensions. Just modelling that element in 3D would be a massive amount of work. Getting its various flex points to animate in a way that both looked even half way decent within the game generally and also a close approximation to the original would be a massive undertaking. Animating a hawk in 2D is a relatively easy affair: you need just a handful of 2D frames frames for each of a handful of animations, and then you flip them for when the hawk is oriented in the other direction. In 3D, the problem becomes much more complex, because characters and elements aren't defined by a sequence of animation files but rather by a rendered model that operates within the environment.
My experience of the average fan of this genre is that they don't really spare much thought for these complexities when they say this or that kewl character should be a guest--and I wouldn't expect every player to have the requisite technical knowledge to understand just how complicated the task would be, but I'm often amazed at just how completely people stop to think about the process whatsoever; I've said it many times here and its as true in this context as any: most people don't take very much time to understand, even on a basic level, how the media they consume (even when they are obsessed with it as an abstract matter) and this leads to a lot of unrealistic expectations, demands, and predictions. But you know who does think about these things, from the second they start considering the concept of a guest as whole? The devs.
And honestly, I think Haohmaru is about as far as they will ever be willing to push things in borrowing from a 2D game with very different movement styles and and animated visual style: Haohmaru at least has non-complicated proportions that can be readily generated within the SC engine, with moves that have a scale and scope that translate decently well for SC purposes. But even with the easiest choice they could have made from SamSho, there are still going to be disconnects. For example, the fact that Haomaru will be jumping at something like 1/8 of his normal jump height, and just generally probably be a little more earthbound, with faster stepping and slightly slower attacks; but even with the most adaptable of the options, those differences will add up to a situation where the individual moves are just not going to look quite right to the SamSho fans. Now obviously PS have a lot of experience at figuring out the middle ground here (and Geralt and 2B prove that the present team is as good as that as any of the other previous teams), but I think this choice begins to press upon the limitations and demonstrates just how constrained the choices for guests really are. Because ostensibly Haohmaru looks a damn lot like another character already in this game, on a superficial level. And yet, despite this, the difference between engines, gameplay, visual style, and dimensionality already begin to make such a similar character difficult to implement in this franchise.