What does it mean when someone says, "This pokemon NEEDS an evo"? I suspect most people are referring to evo's along the lines of the new DP evos. I think those evos could be split into two categories. First you have "natural" evolutions like Electivire, Magmortar, Rhyperior, and Porygon-Z (there are a lot more). These are pokemon that became more powerful versions of their previous forms. They have the same typing and same general purpose. The other evos were "unnatural" evolutions like Froslass and Gallade (that's all, I think). These pokemon are basically new pokemon. I don't know why they are called "evos" of their previous forms, because they really are different pokemon concepts entirely. They have different typing and different roles entirely.
Most of the "This pokemon NEEDS an evo" suggestions seem to be falling in the "unnatural" category -- because everyone is suggesting to alter the typing of the pokemon and their purpose. I'll point out that there were very few of these evos in DP, and most were of the "natural" variety.
I'll also mention that creating an evo of less-powerful pokemon can "ruin" them, in a way. Remember, outside the Smogon world of competitive battling -- Pokemon is a "collectors game". You collect as many pokemon as you can.
Farfetch'd is, and always has been, a "Collectible Pokemon". It's purpose is not to kick ass competitively, or in-game for that matter. You are supposed to do the necessary steps to find and acquire this rare and interesting pogeyman. It's stats, moveset, and battling usefulness is almost irrelevant. As cool as it seems to morph Farfetch'd into a badass Fighting/Flying machine -- it's really not the point. I suspect many true pokemon collectors LOVE Farfetch'd just the way it is. Making collectable pokemon into battle pokemon doesn't fit, IMO.
My statements about Farfetch'd apply to many of the pokemon being mentioned as "Needing an evo". The only popular suggestion that doesn't fit that mold is probably Banette. I don't think of it as a collectible pokemon (it has an NFE form). I think it's a pokemon that was intended to be decent for battle, but is just fail in too many little ways. A "natural" evo of Banette could be cool. But, if I really want a Ghost/Fighting pokemon (and I do want one badly), I'd rather just design one from scratch. If we want it to have "inspiration" from Banette, there's nothing to stop the designers from doing that.
Remember, Cyzir designed Syclant as a possible evo of Scyther.