MTG Salvation

I love this guy. Games play out differently each time, but are enjoyable no matter the opening hand. I went with a spell-centered build with some creature support, and I have left enchantments and other artifacts out of the list to take advantage of Riku. After some play and vigorous trading, this is close to a final version as I'm very happy with how it performs.

All Suns' Dawn.

it's a ridiculous card in any Riku deck!

Hi there. I too have dabbled into the realm of Riku, since it's just soooooo fun copying spells and creatures in multiplayer. It is inherent virtual card advantage, which is quite powerful. One of my buddies has a Riku build full of tiny infect creatures, like Blighted Agent (which he copies with Riku), and then he pumps them up with instants during combat, like Blood Lust, which, when copied immediately kills the opponent with poison counters. He wins out of nowhere all the time, although this is not the build I prefer to run.

Off the top of my head, I've enjoyed playing all of the following creatures with Riku:
Kiki-Jiki, Mirror Breaker + Pestermite -- I usually don't like to win with a combo when I play Riku, but it is nice to have a last ditch effort if you're about to lose. Also, Kiki-Jiki combos with lots of other creatures, and Pestermite's ability to tap something comes in handy now and then as well when you have these creatures on there own.
Glen Elendra Archmage -- is solid, giving you some countermagic with legs that can be copied.
Mnemonic Wall -- is your second Izzet Chronarch.
Steel Hellkite -- helps give you some extra removal.
Sphinx of Uthuun -- is your second Fact or Fiction, which you should probably run if you have it.
Consecrated Sphinx -- is an awesome staple for a reason.
Woodfall Primus -- is usually worth it with it's ETB removal, and it has persist.
Vorinclex, Voice of Hunger -- is a bomb, if you want another one.

As far as enchantments and artifacts go, I agree with you that they can be a bit lackluster with Riku. However, a few are still really good in Riku, like:
Birthing Pod -- if you're running enough creatures, this is awesome.
Sylvan Library -- is worth running, especially if you have some fetch lands.
Mirari -- if Riku ever gets tucked, you'll love that you still have this for your instant and sorcery spells.
Neurok Stealthsuit -- offers some protection for Riku.

For instants and sorceries, I run the following in my Riku build:
Into the Roil -- can cantrip if you want it to, and bounces troublesome permanents for you.
Voidslime, Spell Crumple, Hinder -- a little countermagic goes a long way sometimes.
Fact or Fiction -- is a great spell, and is awesome when copied with Riku.
Skyshroud Claim -- is awesome ramp (the lands don't come into play tapped!).
Bribery -- live the dream by copying the spell AND the creature that you steal into play.
Green Sun's Zenith, Chord of Calling -- once again, both these spells and the creatures you grab can be copied.
Time Warp -- won't win you any friends, but maybe that's not a big deal.
Recurring Insight -- is also usually worth it.
Knowledge Exploitation -- may be worth it, since you can copy it and the spell you steal, since you play the spell, and Riku's ability will trigger.
Eternal Dominion -- when copied is ridiculous.

Good luck.