When you say powder only, you are going to get recommendations for exactly that. That'll usually mean very wide, some form of "rocker" maybe even some degree of reverse sidecut.
Look at this deal as a recipe..........you start with a basic structure like say a Line Prophet 100. This is a wonderfully nimble feeling ski that does a nice job of most conditions much like the dearly departed version of the Gotama and of of course several others. This is not a powder specific ski, it is just a wide general purpose tool. Feels like a "normal" ski.........just big.
So then you start to modify the recipe according to what you want.
(1) Add a little width (Say Blizzard Argos @ 105mm) You'll gain a little lift, lose a little quickness, everything else feels fairly similar.
(2) Add a little (mild) tip rocker (say K2 sidestash @ 108mm) You'll gain more maneuverability, lift and forgiveness in deep stuff, but the front ~~ 20% of the ski starts to feel a little disconnected on firm or rough surfaces. Not too bad, just an odd feel at first.
(3) Add a little mild tail rocker too but keep it flat underfoot. (Say Blizzard Answer @ 110mm). Maneuverability, and forgiveness in the deep gets better yet but now the back 20% feels a little disconnected as well. Again, not too bad but definitely different now.
(4) Now, make the ski a mild continuous rocker tip to tail. (Say 2010 Gotama @ 108mm) Theoretically, this is more maneuverable yet than the previous ski but in this case, I'm not sure that the very mild full rocker adds anything at all in deep snow (over #3). The drawbacks outside of the deeps get more even pronounced in this case. This is the one stop along the road that doesn't make much sense to me.
Now throw in a cup of width........nothing changes much except as noted in (#1)
(5) Now....Increase the amplitude of the rockered portion while keeping the ski flat underfoot and add some early taper to tip and tail. (Rossi S7 @ 115mm) Now you have a full powder specialty ski with dramatically different capabilities in deep snow and again, notable shortcomings in other terrain and conditions.
Of course there is flex to consider and I've really just mentioned skis in the "medium" flex range. And of course you can add ever more and more width (as much as you want really). All these factors are part of it and none are a free lunch as all have positive and negative effects. How you want the cookie to turn out depends upon your tastes and choices of ingredients.
Like chocolate chips vs. raisins........................