A lot depends on how close to modern stereotypes you go, which seems to be what standard kitchen-sink fantasy is running on: exaggerations of exaggerations of exaggerations of templates set many decades ago in a couple classic movies.
For example, this guy is one of the earlier depictions of a werewolf, but he's just as easily a ghoul/zombie, an ogre, a slasher, or a Hills Have Eyes style cannibal hillbilly in modern terms, while being not quite any of those things:
He doesn't have silver bullets or full moons, and he doesn't have An American Werewolf in London style special effects. What he does have is a roughly human body, twisted somewhat into bestial form, wearing rags, which is a great place to start for all sorts of these stock creatures of the night - maybe not quite as gaunt as he could be for general-pupose hungry dead use as well, but we've got a solid enough foundation there.... To say that he has a roughly human face to go with that body doesn't really do justice to what we really have here: yes, that's a human face, but a human face which has fallen far from humanity, replaced with a bloodthirsty madness. You can do a lot with five human heads of that sort, on gaunt, ragged bodies, or on Cannon Fodder bodies, or Dark Age Irish or Late Roman bodies.... The same gaunt, ragged body with a Max Schreck style Nosferatu or ghoul head, or wolfman head, or rotten zombie/skull head, or tormented ghost-face works just as effectively, and all of the above can be mixed-and-matched into the same fomorian horde of creatures of the night....
The questions, I think, are....
Do we keep playing on the same monster cliches everyone else has been doing, only more so, or stop copying D&D, Warcraft, Warhammer, and Hollywood, and go back to the source material a bit?
Can the market even handle going back to the source material anymore?
After all, it's been such a long time since ghosts, ogres, witches, goblins, elves, fairies, ghouls, vampires, and werewolves split off of the original creature-of-the-night templates, I'm not sure modern gaming can shake free of what these creatures are 'supposed' to look like, based on maybe a century in some cases of the popular culture versions.
But, I think a back-to-basics approach is one whose time has come: when flipping through some new "old school renaissance" retro-RPG books, the art is full of creatures that don't fall into easily-identified fantasy cliches, and I might be in a pulp horror "echo-chamber", but it seems to me that the back-to-basics approach has started catching on more and more in horror gaming, too. I think some gamers, at least, are ready to go back to the drawing board, where a fresh, new generation of monsters can be drawn from.
I think the chief hold-outs here might be zombie-apocalypse gaming, post-TSR D&D/Pathfinder, and GW's Warhammer, and I might be imagining things, but the fortunes of some of these holdouts might be changing, and no longer need to hold as much sway over how things are "supposed to look/work/act/be" than they once had....
For zombie-apocalypse gaming, I suspect a 60-figure Modern Zombie Army Builder solution that supplies more stock modern zombie bodies and fewer heads and arms than a typical WGA set would contain (10 small sprues, each with six shambling bodies-with-attached-arms and 12 heads), to be more valuable to zombie apocalypse gamers than something like, say, a 30-figure French Infantry (1914-1940) style set, except with zombies that have been fudged to work in both fantasy and modern zombie games, and given more head and arm options than the subject really demands (6 or more sprues, 5 posed bodies each, with multiple separate arm options and dozens of heads, with perhaps some extra half-sprues with additional options....)
Kitchen-sink Classic Fantasy gamers, I think, are in a very different situation from zombie apocalypse gamers, and could better make use of a less specific Classic Fantasy set.
I THINK you could probably make a one-size-fits-all generic zombie set that would work for both kitchen-sink fantasy gamers and modern zombie apocalypse gamers, but I think doing so would demand so many compromises, it wouldn't make anyone particularly happy. And, I THINK you could make a fantasy zombie-only kit that would be identical to the ones being made by everyone else, which would make anyone building a fantasy zombie apocalypse happy, but not much of anyone else, with or without a separate modern zombie apocalypse set.
But, for best results, I'm really thinking the modern zombie apocalypse stuff might best be kept separate from Classic Fantasy, probably as an Army Builder set, while classic fantasy might best served by favoring variety of fantasy monsters that could be built and used in other settings, over the number of standard-issue zombie apocalypse zombies that could be from setting-agnostic parts....
And I don't think there's any reason that parts from a Modern Zombie Army Builder set and a Classic Fantasy Creatures of the Night couldn't be interchangeable with each other! There's no reason that a carefully-designed Modern Zombie Army Builder couldn't work just as well in a fantasy setting to build a fantasy zombie horde, or an equally carefully designed Classic Fantasy Creatures of the Night couldn't be used to add a little variety to a modern zombie horde, or that heads coldn't be swapped from one kit to the other, or heads from both used to convert historical figures or Death Fields figures into undead. The two kits would simply serve very different niches, by design: fielding large numbers of generic shambling zombies on one hand, vs. a smaller assortment of more customizable ravening undead/fomorian bogeymen on the other....
As for a zombie-head sprue for converting other figures, I'm not necessarily opposed to such a thing, but the more I think of it, the more I'm beginning to think that the target audience for such a thing would be limited: modern zombie apocalypse gamers would, I think, be more invested in putting a load of generic modern zombies on the table, and an Army Builder would provide them all the zombies needed (as many as 60!) to do that, with lots of extra heads to stick onto soldier bodies, for example, for zombie soldiers. I'm not sure they'd be interested in a sprue of heads without bodies for that purpose (but I'm willing to be surprised if I'm judging their market wrong!)
Meanwhile, fantasy gamers would be in a similar boat: they could kitbash spare zombie heads onto the bodies of Dark Age warriors or knights or whatever, but could always find uses for creatures-of-the-night bodies in rags, too, while most would be wanting complete zombie bodies to build from.
So, I'd be surprised if a zombie accessory sprue would have much demand. I'd buy a couple, but my tastes are possibly a bit odd compared to the larger zombie customer demographics!