(There's a lot of text with explanations coming first, if you want to just know what I'm changing you can skip to the Ki and Kit Changes, but please don't reply without having read the reasoning behind the changes)
I know what you're thinking. "But he has a Riven icon/But he is Leyrann", and "This is never going to be a good rework, that b*tch needs to be nerfed, not buffed". Well, I'm going to reason up every change, and hopefully convince everyone that this is doing it right. Oh, and before I forget, no I'm NOT trying to buff her.
Before I'm going to imitate Riot, I'm going to have a look at her statistics : They're quite clear. Riven has a win rate incredibly close to 50%, and in fact is under it more than above. Now, this may be lowered by .5-1% because her current high popularity and people who can't play her playing her anyway, but it looks pretty balanced.
BUT that doesn't mean she's a good champion for the game right now. Riven has a 16% ban rate. And there's a reason for that. Take one look at the forums and you see the problem: Everyone. Hates. Her. That shows she's anti-fun to play against (did I say I might sound like a Rioter? I might sound like a Rioter).
The combination of above two points makes it clear what has to be done: Keep Riven's strength at the same level while giving her more counterplay than she currently have. I think I'm going a bit too fast now though, because where does she need more counterplay? In other words, where does she lack counterplay?
People often say her resourceless shield or her being resourceless overall, but that can, as I'm going to explain too, be solved easily. The problem lies in her ult. Problems, actually. The first is, that she has this huge, 15 second window of increased power. The second is that her Wind slash is a problem in teamfights. Both can be solved. Before you can come to a good solution, you first have to fully understand the problem, however. For that, we have to dive deep down into all the Riot ramble you see with every rework. Simply because a lot of things they say are true, even if you believe they're applied wrong.
I'm going to start with fleshing out why Riven originally recieved Wind slash. As you can read in the original talk about her design, inspiration has been drawn from fighter games, and what does every character have in the average fighter game? A finisher/execute. Riven also gained that, and it's a perfect ability, requiring you to deal damage (at melee range) before it's useful. Except... League of Legends, contrary to fighter games, is a team game. If someone else deals that damage for you, preferably with some cc, you just double tap R and get a triple kill. That's not fun for the enemy team, and it shouldn't be there, most of all because you can then still finish off the others with your bonus range and damage.
Changing the ultimate
Riven (and mostly her ultimate) needs to be changed, but as I proved earlier already, she doesn't need nerfs. You can nerf the ult damage and buff other damage, making it useless. You can cut it's range, again needing to buff other damage, again making it useless, etc. Let me just say that if there's anything I hate, it's a useless ability. Instead, let's REMOVE it and replace it with something else. Considering Riven's core design should, imo, be left intact, she should remain a combo champion, and she should still have an execute. But it should work totally different.
First of all, I'm going to introduce a resource. You probably already know of my idea to give her a Ki resource, but the way I'm going to apply it is new. With enough Ki, you can buff an ability, giving it bonus effects. Now, let's see how it works:
The ultimate is going to be totally changed, and will work with a new mechanic, which no other champion has. It has a certain cost of Ki (numbers coming later in this post), but it's no requirement. If you don't have the required amount, you gain the ultimate mostly like now, except that it builds up Ki over time while it's active (with a maximum duration) and you can reuse if it you have enough Ki, like the other case works. If you activate it this way, it goes on a high cooldown after the second activation or the time is up.
If, instead, you activate it with enough Ki, you buff up your next ability to turn it into a sort of execute at the cost of Ki. This puts the ultimate on a shorter cooldown.
Ki gains and losses
To avoid people stacking up Ki and then walking into lane being able to use a hugely buffed ability, it has to be designed in such a way the only good way to get it is from fighting champions. Let's just put some numbers out here:
Maximum Ki: 200
Ki needed to buff an ability: 140
For every point of Ki Riven has, she gains 1% more Ki from all sources, but loses 1% more when it decays. This encourages trying to get your Ki as high as possible before using it.
Ki gained per second when in ult: 5 (this means that she cannot gain enough Ki to use a buffed ability if she starts from 0 and does not fight)
Ki gained from hitting Q on minions: 2 per minion. Nothing when also hitting champions.
Ki gained from hitting W on minions: 5 per minion. Nothing when also hitting champions.
Ki gained from hitting Q on champions: 5 plus 5 per champion.
Ki gained from hitting W on champions: 15 plus 10 per champion.
Ki gained from hitting enhanced autoattack on minion: 2.
Ki gained from hitting enhanced autoattack on champion: 15
Ki gained from hitting normal autoattack on champion: 5
This means a full combo will earn you (when hitting one champion) about 120-150 Ki, depending on how many autoattacks and abilities you actually hit. I haven't calculated exact numbers due to the gaining 1% more Ki per Ki she has making that nearly impossible, but without it she would gain 115 over this full combo. Also keep in mind that she cannot use her Ki if everything is already on CD, and it also decays:
When Riven does not damage an enemy champion for 3 seconds, Ki starts decaying, with a speed depending on the most "important" thing that happened in the last three seconds (the more important, the lower the Ki loss):
When being hit by a champion recently, Ki decays at 3 per second.
When having hit a minion/monster recently, Ki decays at 5 per second.
When having been hit by a minion/monster recently, Ki decays at 7 per second.
When totally out of combat for at least three seconds, Ki decays at 10 per second.
We're finally there. The last point. The kit changes. Or additions, mostly.
Passive: Remains the same. secondary bar shows Ki (obviously).
Q: Remains the same. Bonuses when used with Ki:
(note: Riven can only empower one Q charge, not all three)
First or second Q: Gains about double the current range (maybe a bit less than that) and deals an additional 50/125/200 (+0.6 bonus AD) damage. Also stuns any enemies hit for one second.
Third Q: Leaps into the air and lands at target place no more than 500 units away (a bit less than Aatrox Q), knocking up enemies for 0.75 seconds (about twice as long as currently I think; that's what I intend at least) and slowing them by 30/35/40% for 3 seconds.
W: Remains the same. Bonus when used with Ki:
Deals 50% increased damage and stuns for one second instead. If Riven autoattacks a stunned target, the autoattack deals 70/140/210 (+.5 bonus AD) bonus damage (on top of the passive). Cannot crit, and Riven can do this only once.
E: Now costs 15 Ki when not used in the direction of an enemy champion (in the direction means less than 90 degrees away from the champion). Cost does not apply if coupled with the Ki bonus, if you don't have the required 15 Ki it instead costs you 3% shield strength per Ki you lack (up to 45% reduced strength). Bonus when used with Ki:
Has 650 range instead (compare: you would just be able to autoattack Caitlyn if she was autoattacking you from max range, or you would stand exactly on top of Jinx (short range) doing the same), shield lasts 2.5 seconds instead of 1.5 seconds. Gains 40/100/160 (+0.4 bonus AD) extra shield.
R: Total rework. After taking at least one point in it, you can buff an ability with Ki, granting it bonus effects to strengthen it. Cooldown when used with less than 140 Ki: 160/130/100. Cooldown when used for the Ki effect: 55/45/35 seconds. Both cooldowns start counting after you use the buffed up ability.
Passive: Grants 5/10/15% bonus AD when not on cooldown.
When used with less than 140 Ki: Gain bonus range like currently, gain 5 Ki per second, passive bonus increased to 10/15/20%. Lasts up to 15 seconds. Can use again when you have over 140 Ki to buff an ability, but this ends the ulted state and makes it go on cooldown after usage.
When used with at least 140 Ki: Buffs up an ability. Gain the buffed up state for the usage of the ability, but NOT for any autoattacks you may use in between.
A few more notes
Because I can't stop writing.
First off, I want to point to Riven's most important (supposed) weakness: Total lack of range. This rework further emphazises that by taking away her only over-300 ranged ability and only allowing her to dash long distances when she's already closed the gap recently.
Next, don't forget the buffed abilities are ultimate-level, and supposed to be execute-like, except her E (though you can use it offensively if the enemy flashes away or something like that). You can only get one of them, and only after nearly a full combo.
All cooldowns are with a maximum CDR build in mind.
This makes her even more vulnerable to ranged harras, because she will, if she doesn't have Ki from fighting you or pushing the lane (two things she's probably not going to manage/want), she either has to dash towards you to get the shield, opening up for more poke, or dash away and have a severely weakened shield.
In general, this rework is meant to keep Riven's strength the same while giving her more counterplay, by making the build-up to a huge, powerful ability clearer and yet giving her tools she didn't have before to outplay opponents if she plays it right.
If anyone wonders, yes, Ki buildup exists before level 6. If you turn 6 while fighting, you should be able to buff an ability just as fast as anyone else is able to ult (if you have the Ki, of course).