Creature types are by and large a flavor addition to a typeline, except when a block has tribal themes, but they play an important role in creating the feel of Magic. There are races like Elves, Goblins, and Zombies that always show up, of course, but there are also quirky types - one-offs and odd races - that don't get nearly as much spotlight time. But those creatures always stand a chance of coming back, as Doug Beyer knows, and confirmed on his Tumblr blog recently.
When captainjackharness asked him, "Are we likely to see Kithkin or Scarecrows again? I loved Lorwyn, but I get the impression it wasn't super well liked," Beyer responded:
Kithkin and scarecrows have their fans. Sometimes there are elements of the game that are not as popular at the time they’re around, but over time there’s a kind of nostalgia curve, and people warm to them after the fact. It’s kind of fascinating.
I think scarecrows are great — they’re just waiting for a world where they’ll make sense. And I think the idea of kithkin appeals to people, as do halflings in D&D or hobbits in Tolkien — we just have to figure out what role kithkin should have in Magic as a whole, and what visual portrayal gives them the best shot at wide appeal.
Cutesy pumpkin-headed humanoids have a hard time being center stage, maybe. So having them be the human analogue on a world without humans might not have put kithkin in their ideal role. Luckily we have infinite more Magic sets to make, so we have many chances to try other things in the future.
Kithkin have an uphill battle to fight if they're trying to become white's characteristic creature type because they simply can't be taken seriously. The whole point of them is they're underestimated - simple country bumpkins who turn out to be great at war when hearth and home are at stake. Still, dmmyccnt said in the comments, "Kithkin were at their best when they were a little alien on Shadowmoor. Turning the thoughtweft into a creepy, xenophobic hivemind was the perfect way to showcase the strengths and weaknesses of white. It actually made hobbits interesting."
Meanwhile, Scarecrows excel at the "uncanny valley" - when the artifact creatures of a world should be emotionless and creepy, they're one of the creature types to call in.
Oddly, the Lorwyn Kith's pseudo-hive mind of the thoughtweft and the plane's tatterdemalion assimilation-happy Scarecrows would fit quite well with a Phyrexian invasion; though this is certainly unlikely in terms of full block ideas, it'd make for a heck of a supplemental product plot.