He_Who_Hungers Deckling

It looks to me like you're going for a sort of different type of deck than mine (or at least are playing it in a very different environment), so I'll be careful with suggesting too much, and stick to the things I have found worked well in general for me.

First I would recommend is Channel the Suns. If you're looking to get the King out as early as possible, this is the card to do it, allowing you to drop him as early as turn 2 in combination with Sol Ring. Even beyond that, it usually feels pretty good to get him out a turn earlier, or for one less mana, sometimes enabling you to follow him up with a scarecrow right away.

Secondly, I agree with your aversion to use too many changelings, but if you're looking for a wacky combo, it doesn't get much better than the trio of Changeling Hero, Changeling Titan and Changeling Berserker. I find they work great on themselves, as they allow you to champion a scarecrow, which re-enters when they die, and can get you another trigger. If you manage to get all 3 of them together, though, you can have them champion each other (e.g. play the Titan first, champion any creature, then drop Hero, champion the Titan, then play Berserker, champion Hero. Since Hero leaves play, Titan re-enters, which champions Berserker, causing Hero to return and so forth), which is usually a game ender since you're able to destroy all of your opponents' permanents. I'm usually not much of a combo player, but I personally find this one very funny, if a bit hard to get off.

For theme, I'd urge you to play Reap and Sow. It's not really a great card, but it can help you ramp and tutor for some of the more key Nonbasic lands (in your case probably Cataracts or Inventor's fair). And it just fits the theme. If you're playing Reaper King, you might as well start Reaping (or sowing, or both!).

I also tend to like Tezzeret the Seeker , especially if you back him up with a set of artifact lands (Seat of the Synod, Tree of Tales, Vault of Whispers and Ancient Den). You'll mostly want him for his second ability, which allows you to tutor your artifact lands for some added ramp, or a simple -1 to grab that Heap Doll if you need a RK trigger and don't have one on hand.

Lastly I'd like to recommend either Fabricate or Drift of Phantasms (or both, as I did). The point of fabricate is hopefully somewhat clear, and I tend to like the Drift as it can find a lot of your key cards (I usually end up transmuting him into either a Scarecrone or a Chromatic Lantern), and even if you don't need to transmute it, it can never hurt to have an extra flying blocker.

I'll leave it at that for now. If you're looking for more inspiration, take another look at my list.

Thanks for the suggestions! As it stands, I feel I am still too dependent on my various changelings to make up for the deficiency of most of the artifact-based scarecrows to be able to cut them. I agree that I'd like to be able to run the tribal without them (as I do with Conspiracy and Arcane Adaption), but I feel right now sticking to just the artifact-based scarecrows would be too much of a burden to carry for this deck (although I hope to prove myself wrong in the future).

As a result, I have cut most of my heavily artifact-dependent synergy, as it felt like it didn't come into play often enough due to the fact that part of my creature base did not consist of artifacts.

Obviously Door of Destinies is a great card for any tribal. I personally just am not a great fan of it. On top of that, raw power is usually not much of a factor when I play this deck. Usually the King himself is public enemy #1 and if I manage to make him stick on the board, I don't have too much of a problem controlling the board and closing the game out (hitting for 6 commander damage usually stacks up pretty fast as well).

In addition to the above, I have just updated my list and want to see what the effect is on how the deck plays before I make any more. I will definitely take a look at your list some time and let you know what I think!

+1 for the inclusion of Booby Trap

If you're looking for ideas, take a look at my list: A song without music.... It's in a similar spirit (also the result of the dismantling of my Sygg deck), but with a slightly different flavour, mine seems more aimed at getting my ninjas through succesfully and capitalizing on that with enter/leave the battlefield triggers, whereas yours seems more focused on capitalizing on Yuriko's ability.