Have you considered putting a copy of Fork and maybe a Laboratory Maniac in case you run into a doomsday deck? Or else there's a combo with Dualcaster Mage and Ghostly Flicker which makes infinite mana, in addition to Riku it makes infinite 2/2 dualcasters. Also, the spell Eldritch Evolution will fetch a lot of your creatures to combo out on turn. If you copy it, also, you don't have to sac an additional creature since that's the casting cost and not a side-effect. That's the same rule for Harrow which will net you three lands for the price of two.

May I suggest Sunbird's Invocation from Ixalan?

I have found it to be great in Riku and Maelstrom Wanderer

SirSh4ggy: I'm trying out Eldritch Evolution to see how it works out! I've been contemplating this card since it's release and you made me pull the trigger. Thanks! As for Fork and Laboratory Maniac, I don't like to rely on others spells for copies and generally Rikus ability is more than enough for copying my own. As for lab maniac, I just really dislike the card and refuse to play it. Now for the Dualcaster Mage + Ghostly Flicker combo, I'll have to think on it. With Palinchron's inf combo I only need riku, where this one takes a bit of hand sculpting and I would need something to win with in hand or it's useless. I do have to play around with it before I say yes or no.

xcver: I enjoy Sunbird's Invocation so much as a card, but I can't play it for the same reasoning as why I don't play Mana Reflection. It's a 6 CMC enchantment that I'll probably have to tap out (or close to) for and will have to just hope it lasts a turn so that I can use it. And if I do get to use it, that'll be devastating for the board, but it will draw the eyes of everyone on table too. I'll test it out to see, I could always be wrong!

Unless of course you have Leyline of Anticipation out :) You could even double up with Vedalken Orrery

On regards to Regrowth. I understand that it is more cheaper, but in terms of flexibility Wildest Dreams is wildly superior. Since 5+ mana is an issue I will not fervently suggest cards like Restock, Creeping Renaissance, Praetor's Counsel or best of all Seasons Past

But back to serious, what about Solemn Simulacrum or Coalition Relic?

I always forget Vedalken Orrery is a card. Flash does make this deck more potent, but I need to test to see if the redundancy is worth the slot.
As for Regrowth, this card is actually perfect for what I need it for. I almost never want to grab more than one thing from my grave and I generally just grab the second best (which is usually ramp or a counter) with the copy, or don't even copy it at all. This deck is meant to roll with the punches. "Oh damn, you stopped the Palinchron combo. Guess I'll just have to find another way to win." The graveyard doesn't really matter in this deck. It's basically another exile until I get Regrowth/Eternal Witness or am trying to hide that I have Devil's Play in there. It also helps a lot with piloting because you can set up 2 (or 3 if you're lucky) game ending combos in one turn. Nobody has that many counters..
Sad robot should really be in the deck. I always look at it and say "hey, this needs to be in Riku" then flip the page. I really do need to find a slot for it. And for Coalition Relic I'd probably play Chrome Mox over it. It's a Phenomenal ramp card and I do think it deserves testing. It also doesn't exile a card out of my hand.

Thanks for all the suggestions! Keep em coming, I love having to re-evaluate why I do play certain cards and have things to test out.

Hey hey nice Riku deck! Definitely seems like the quintissential roll with the punches riku deck. He can draw a lot of hate if he gets too active too quickly, I've found. I like that you have the option to end things several ways or copy buffers that make people want to stay away from your creatures. My deckbuild is fairly similar but I use a few cheesier removal spells because of the color combinations.

I like building Riku decks where I don't know what's EXACTLY going to happen but there is enough good stuff where I can either effect the board or, like your friends say, go from no board state to "I win" haha. Good counterspells make that happen, counterspells on bodies are even better.

I'm actually looking to update my Riku deck but I am torn as to which route to go. I have been collecting some eldrazi and thought that could get a little ridiculous. He has such explosive ramp it wouldn't take much to pair down some other things to try and rush and drop eldrazi. Once you get to the point you can produce the mana you should have plenty of buffers out to protect Riku and your other creature drops.

The other option was using some of these fun dinosaurs from Ixalan and Rivals. There are a bunch of very vanilla ones, but as a "tribal" they work pretty well and some of the Enrage mechanics would be good in EDH (especially doubled with Riku).

I'm really sad this deck's out of my budget because it looks like a blast to play. +1