The average income per game is 11,000 gold per player, the average game length is ~33 minutes.
basically 3 major items and boots.
Having a full 6 items is more detrimental than just sticking with 3. If you're planning on having the power of some you threw in at the end, that's bad. Also - if a game lasts long enough to get more, I've found that adapting to the enemy and the flow of the game is what tends to happen for those last couple of purchases. Even on an otherwise glass-cannon champion you should still pick up 1 major defensive item - as bolstering you defensive by a solid 50% is considerably better than enhancing your DPS by ~15%, especially when burst-casters can safely 100%-->0% you. Defense should always be reactionary.
Also - 3 major item core builds break down nicely into early, middle, and late game - so you can concentrate on the order and build-paths over cobbling together some useless amalgamation with synergies strewn out at unusuable and awkward times.
Like really - why would you get 2 AoE wave clear items in both Shiv and Hydra, and why are they on the end of the build? CS is super important during the lanning phase, it's useless when you have 6 items.
As for Riven - her considerable AA re-sets make %AS not ideal on her. Roughly half her damage is from her physical damage abilities, and she scales a defensive shell with raw AD.
Physical damage skills are only enhanced by AD and armor penetrations - so it stands to reason stacking AD and armor pen is what you want to do on Riven.
The Brutalizer is absurdly strong for it's cost, so a Brutalizer upgrade is ideal within her first major buy.
Hydra is also solid for the sustain offered and essentially AD lich-bane active it has, and while more wave-clear is fairly redundant on Riven, it still helps early game.
Black Cleaver and Last Whisper are Riven's core damage multipliers for her kit, so both of those should be grabbed pretty much 100% of the time.
Hydra and Sword of the Divine are the best secondary damage multipliers on mega-AD builds, as both are essentially a different take on Lich Bane. Considering Riven's primary defensive mechanic, her shield, actually scales with AD - the mega-AD item in Hydra is the clear winner there.
Lucidity boots are my personal favorite choice on her, as you actually get a bit of defense through shields and mobility happening sooner. In top lane counter-building your opponent directly can be increadibly advantageous for trading potential, so Ninja Tabi and occasionally Mercury Treads can also be handy. Really depends on the match-up.
(maybe swap teleport for barrier to provide plays with the E)
Items:
(speed is important, both in recalling and chasing things down to pay for recall)
(damage is damage, and life steal is fantastic)
(more damage, provides potentially humongous burst)
(again, the damage and some more crit potential)
(pierce damage on all abilities)
(massive wave clear, more burst, more dmg, more life steal...just fits)
