Made a few quick changes, mainly to add more ramp and card draw.
-Kodama's Reach, +Harrow: Another Riku player pointed out to me that because sacrificing the land is part of the cost, if you copy it with Riku, you don't have to sacrifice an additional land. You wind up ramping by 3 for 5 mana, and getting 4 untapped lands that turn, which are much easier to put to use than the 2 untapped lands you get back from copying something like Wood Elves.
-Tree of Tales, +Turntimber Symbiosis
Flip: I decided I had enough targets for Trinket Mage that I didn't need the artifact lands. Turntimber Symbiosis is a great replacement for the green one that I happened to have on hand. Card draw in a land slot. Excellent.
-Great Furnace, +Mountain: Straightforward, but worth noting that part of the reason for it was that this deck can run out of basics quickly with all the fetching it does.
-Seat of the Synod, +Tolaria West: I realized if I ran this, I could cut Expedition Map. It and Turntimber both being taplands does raise my eyebrows a little bit, but I'll keep an eye on it in playtesting.
-Expedition Map, +Thought Vessel: Even though I have a bit of a land toolbox, Expedition Map was mostly here to grab Reliquary Tower. With Map, Tower, and Trinket Mage, I had three cards that could lead to having no maximum hand size. Even though Trinket Mage won't lead there anymore without the map, Thought Vessel, Tolaria West, and Reliquary Tower still fill three slots. Thought Vessel is also another piece of 2-mana ramp. I prefer ramp that can be copied by Riku, but for such a good effect, I'm willing to run a piece that isn't. Plus it's not replacing anything that is.
-Creeping Renaissance, +Regrowth: More versatile. When I cast Creeping Renaissance in this deck, I usually only need one of the cards anyway.
-Coiling Oracle, +Nature's Lore: Improves consistency, and synergizes with Eternal Witness and Regrowth. The other additions more than make up for the lost card draw slot.
-Keruga, the Macrosage, +Escape to the Wilds: When I put Keruga in, I was thinking this deck would be heavier on ETB creatures than it wound up being. There are still some, but not enough for Keruga to be consistently good. Losing the body is a bit of a shame, but Escape to the Wilds is such a phenomenal card that it's worth it.
-Cultivate, +Collective Voyage: Essentially here to act as a 2-mana ramp spell that can be recurred with Eternal Witness or Regrowth. More importantly, it curves into Riku earlier than Cultivate does, and scales to help ramp back up to casting Riku again if he gets removed too many times.
-Eureka Moment, +Inscription of Insight: More versatile, acting as situational removal or a body (two with a token doubler) that can be quite big.
-Inferno Titan, +Incubation / Incongruity: I have plenty of other, stronger win conditions. Incongruity is more reliable as spot removal, and Incubation adds card selection that's kind of like a mini Dig Through Time when copied with Riku.
-Evolutionary Leap, +Humble Defector: Another card that looked better when I thought the deck was going to be more creature-based. Humble Defector is less likely to sit on the field with nothing to do, and can be copied with Riku.
-Prime Speaker Zegana, +Uro, Titan of Nature's Wrath: I love Zegana, but she does depend on you having a board presence. Uro provides value no matter what your board looks like, scales nicely with Riku, and can be a threat later. This also frees up my copy of Zegana, which might give me incentive to finally build that commander deck around her that I've been thinking of.
-Return of the Wildspeaker, +Harmonize: Lower ceiling, but higher floor, which is valuable for this deck of mostly small creatures. There are a lot of options for card draw around this spot on the curve, so this could become any number of things, but even as-is, it's quite solid.
-Chord of Calling, +Champion of Wits: Chord is very good, but very expensive, and this deck doesn't reliably get the creatures to convoke it. It can be fine late, but it's not good early. I figured it would be a good swap for another piece of card draw. I'm not positive I want this to stay Champion of Wits, but we'll see how it plays.
-Kiki-Jiki, Mirror Breaker, +Oracle of Mul Daya: Kiki-Jiki was never a win condition in this deck. He's more like a blink engine, and for that, he might come back in later. I wanted to fit in another card draw piece to get up to 15, and while Oracle of Mul Daya isn't exactly draw, it's close to it. I've been meaning to put it in anyway, since it curves nicely into Riku after a 2-mana ramp spell, and does well when copied.