The limit of what can be displayed using Code Page 437 is something that has been acknowledged by Toady, which is why no new creatures were added
I don't see why having no new creatures in 40.01 has anything to do with the ASCII limit.
1) Toady doesn't add new creatures very often. Between the 3D update in 2007 and the big 2010 update that added the caverns, there were no new creatures (unless I forgot?). I don't think there were many (any?) additions during the public Flat Fortress era, either.
2) Although there's new Night Creatures planned for later, Toady gave a pretty good explanation of his plans for all of them in one of the DF talks (there were originally going to be more types in the vampire/werewolf update, but they pushed them back for the sake of development time). All of them will be represented by color-coded squiggly-hat Ns (like vampires, werewolves, and necromancers), and there's plenty of space for them there. Between the planned night creatures, the cavern critters from DF2010, and the vast wodges of regular critters from the animal donation drive, we're pretty much covered on all fronts.
3) Though the scope of 40.01 was fairly broad, everything could be sorted into a handful of clumps of loosely related things that sort of required each other. None of these feature-wads really required a new set of critters to be added, not even the Night Creatures (though I suspect we'll be seeing more of them in in 2016).
But yeah, we're likely to get a graphics overhaul around or after the full v1 release. The first Slaves to Armok WAS in full 3D, and perhaps one day we'll see that in DF.