Implementing this would not require new animations, as the existing ones could simply be mirrored. However, as already mentioned, this would be utterly incorrect for the period. While the ancient Romans appreciated left-handedness as an exciting quality for a gladiator to have this far-sightedness was seemingly lost by the middle ages, when things that were seen as going "against nature" were frowned upon or even feared. Medieval people were far more suspicious than modern people (where superstition often goes no further than believing in ghosts or avoiding the number 13).
I'd also question it feeling more natural to left-handed gamers. A scant few games allow left-handed orientation (Counter-Strike is one I can think of) and hardly any have it by default (The Legend of Zelda series being the primary and probably only example), so to a left-handed gamer it might actually be slightly
unnatural to have a game able to work this way, even though it is actually correct for them as far as the real world is concerned. Certainly when playing Zelda games I never felt that Link held the sword in the "wrong" hand, despite me being right-handed and these being the only games I have played with a left-handed character, and I'm only rarely consciously aware that he is even holding it differently; this may well be the case for left-handed players of right-handed games. I would compare this feature suggestion to those women who occasionally complain (very loudly) about how RPGs are all "male-centered" and that they are "unable" to play them because they cannot "relate" to being a man; when it comes down to it players are often
nothing like the on-screen persona, and this can extend to both gender and handedness.
Another thing to consider is that, if this were implemented, some players (right-handed or otherwise) would choose this option to gain a perceived unfair advantage against right-handed foes, thus artificially increasing the number of "sinister" Calradians.