Oh boy. Riku. What a goofy general. I’m so glad someone in our group is tackling him, can’t wait to get some games in.

Birds of Paradise is sort of hit or miss in EDH. It’s an awesome card, don’t get me wrong- one of my favorites, actually- but look at it this way: in a 99 card deck, you have to consider that you will, statistically speaking, draw early game cards in the early game. Drawing a late game Birds is an awful feeling, especially when you’ve already ramped enough. I ran Birds for a while in Ghave but eventually cut it because it was just hurting the consistency and overall power of the deck.

Mycoloth seems questionable. It doesn’t seem like you’ll have many tokens, barring the extra copies Riku makes but you seem more focused on using the tokens for their ETB effects. He just doesn’t seem like he does enough in here to deserve a slot.

Snapcaster could be questionable as well, but that depends on what your final build looks like. Only have 20 targets is pretty bad in a deck this size. Of course, being able to flash some of these back will be a little bit bonkers, especially if you’ve got enough mana to copy either the Snapcaster or the spell or both with Riku. This may change with this next bit as well:

Gitaxian Probe/Spin into Myth/Hull Breach/Mwonvuli Acid-Moss/Ponder/Preordain: These are all great cards, but I tend to stray away from single use small effects like this. With Snapcaster, this may be different, but you’ll need to do some substantial playtesting to figure that out. One thing I like to consider (similar to the Birds reasoning above) is how much I’ll like a card when I’m running out of gas and am simply down to topdecking. I like cards that will be decent at most stages of the game, rather than just really good at one stage and terrible at another. Some one shot cards that may be worth running anyways: Ancient Grudge, Krosan Grip, or Beast Within.

Gruul/Izzet/Simic Signets: This depends on how much ramp you find yourself needing. I personally don’t like using these; I prefer either more lands or more ramp spells. Sylvan Scrying or Crop Rotation might be helpful. You’re only running 36 lands, I would consider bumping up to 40 since you’ll want the mana to use on Riku’s abilities.

Speaking of lands, some cards to consider: Temple of the False God, Desolate Lighthouse, Grove of the Burnwillows, Gruul Turf/Simic Breeding Pool/Izzet Boilerworks (works great with Avenger of Zendikar!), Karplusan Forest/Yavimaya Coast/Shivan Reef, Reliquary Tower.

Onto your maybeboard cards:

Brainstorm/Clairvoyance/Manamorphose/Snakeform/Spellbook don’t seem worth it to me. Reliquary Tower is pretty much better than Spellbook in every way, and the other ones don’t really do enough (I’ve also seen a crapton of people forget the Clairvoyance trigger, which will only be worse in a multiplayer game).

Doubling Cube seems really good in a deck like this, where you’re really going to need lots of mana. Same goes for Mana Reflection and Plasm Capture.

Voltaic Key probably won’t do enough here. It’s great with a lot of your artifacts, especially the mana producers and Strionic Resonator, but I think you’ll find yourself in situations with the Voltaic Key and nothing to do with it.

Ring of Three Wishes is pretty awesome, but questionable. Really depends on how much you find yourself relying on certain cards to stay in the game.

Staff of Nin is always fine in any EDH deck. It isn’t amazing, but it gives you consistent card advantage and pings your opponents.

Some other staple-type cards I would consider: Maelstrom Wanderer (depends on where you land on some cards, some stuff is pretty bad to cascade into), Chancellor the Spires (makes infinite clones if someone else has played a Rite of Replication), Consecrated Sphinx, Diluvian Primordial, Frost Titan, Molten Primordial, Sylvan Primordial, Woodfall Primus, Wolfir Silverheart, Yavimaya Elder. 

I would consider dropping Harrow.  I understand that the instant speed land searching is good, but you will need all your mana to be copying spells.  I think maybe substituting Kodama's Reach?

Also, Birthing Pod seems a bit out of place.  I understand the ability to curve up your creatures without using mana, and that Riku can copy them, but it feels out of place to have a sac effect where most of your deck does not like to be sacrificed.  Although Traitorous Blood + Birthing Pod is a good synergy.

Replace High Market with Reliquary Tower.  I don't see any way to pump out a lot of little guys to sacrifice for life, nor do I see a lot of sac benefit besides the 1 life.  I could be wrong though.  

Another land to add is Thespian's Stage.  It is a repeatable Vesuva.

Possible Jin-Gitaxis?