I definitely agree with all of the champions you mentioned.. I'll just throw in a few ideas for passives, and get some feedback on what you think.
Ashe:
Focus: (Innate): While not autoattacking, Ashe's critical strike chance increases by 3 / 6 / 9 / 12 / 15 / 18 % every 3 seconds. This bonus critical strike chance is removed after Ashe's next autoattack.
My idea..
Focus: (Innate): Every 4 abilities used, Ashe is able to empower her next ability with an additional effect.
Frost Shot: Not affected.
Volley: An empower volley pierces through the targets hit.
Hawkshot: An empowered hawkshot has the sight properties of a Vision Ward.
Enchanted Crystal Arrow (Ultimate): An empowered crystal arrow will travel significantly faster.
(Just some ideas, the ultimate doesn't have to be including since it might mess up some people who are used to Ashe's arrow's speed)
Evelynn:
Determined Killer: (Innate): Damage received from enemy minions (not jungle monsters) is reduced by 50%.
My idea..
Determined Killer: [I](Innate): Evelyn gains an additional 5% damage bonus after not taking any damage for 10 seconds, increasing to 7.5% at level 6, 10% at level 11, capping at 12.5% at level 16.
(This idea is a little iffy.. But it would allow her to have more of an impact when she comes out of stealth with some bonus damage.)
Leblanc:
Mirror Image:
(Innate): When LeBlanc is brought below 40% health, she instantly becomes stealthed for half a second. When the stealth fades, she creates a Mirror Image of herself that can move, deals no attack damage, has 53 + (17 x level) less health, and lasts for up to 8 seconds. This can only occur once per minute, regardless of cooldown reduction.
My idea..
(Innate): When LeBlanc is brought below 40% health, she instantly becomes stealthed for half a second. When the stealth fades, she creates a Mirror Image of herself that can move, deals no attack damage, has 53 + (17 x level) less health, and lasts for up to 8 seconds. This can only occur once per minute, regardless of cooldown reduction.
(Active): LeBlanc can disable her passive for three minutes to create a Mirror Image of herself that can move, deals no attack damage, has 53 + (17 x level) less health, and lasts for up to 8 seconds. This can only be used once every three minutes, regardless of cooldown reduction.
(I'm not even sure if it's possible to have a passive you can click on to use up.. But if so, it'd be interesting to see how people would use it to deceive people. After all, she IS the deceiver..)
Nunu:
Visionary: (Innate): After 7 autoattacks, Nunu's next spell will have no cost.
My idea..
Ceaseless Hunger: (Innate): After Nunu consumes 4 minions, he gains a shield that blocks up to 10% of the damage done by the consumes that lasts for 10 seconds.
(I couldn't really think of a great idea for Nunu.. But this way, I think you'd be able to save up that 4th consume for a big team fight where you'll need that shield to help tank the opposing team. Also, I feel like this passive idea isn't the greatest because it focuses on one ability..)
Renekton:
Reign of Anger: (Innate): When Renekton is below 50% health he generates 50% more fury from all sources.
My idea..
Aggression: (Innate): When Renekton is below 75% health he moves 4% faster. When Renekton is below 50% health he moves 8% faster. When Renekton is below 25% health he moves 12% faster.
(This idea would allow him to be more aggressive the lower his health is, where as his current passive promotes fury gain and "turtling" in turn.)
Taric:
Gemcraft: (Innate): Taric's autoattacks replenish his mana for 7.5% of the damage dealt.
My idea..
Diamond Body: (Innate): Each time Taric is critically hit, the damage is reduced by 25% and he gains 10% of that damage as mana.
(This would allow Taric to fit the role of "tank/support" more easily, and give him that extra mana in a big teamfight.)
Again, these ideas were all on the spot, and I haven't thought much on the rations/concepts very much.. Which is why I'm posting it here, actually.. Feedback! Tell me what you think, and what could be changed/improved on?