A witch would be cool IMO.
Passive: Scythe rider. Witch rides a scythe, increasing movement speed and allowing her to pass over minions/enemies (Janna passive basically).
Q - Passive: Clarity - basic attacking applies Clarity to the Witch, stacking up to 5. For each stack of Clarity on the Witch, she deals X additional magic damage (up to X*5 at 5 stacks). Using abilities consumes her Clarity stacks to provide additional benefits.
Q - Active: Alacrity - for the next 5 seconds, allied target gains bonus attack speed, and attacks apply a 10% stacking slow (2 second duration, refreshed on new stacks), 20 second cooldown. Can apply to herself. For each stack of Clarity consumed by Alacrity, the Witch regains X mana.
W - Wilt - After 1 second, deals damage in the targeted area. For each stack of Clarity consumed, Wilt restores X health over 5 seconds.
E - Cull - Witch quickly dashes forward on her Scythe, and passing over small walls (like a Nidalee hop, maybe a bit further though). For each stack of Clarity consumed, Cull stuns targets it passes through for 0.3 seconds.
R - Revive - The Witch returns to life the most recently deceased ally (within a certain distance). If the Witch dies while Revive is available, the Witch automatically Revives herself from death. For each stack of Clarity consumed, the ally returns with an additional 20% of their max health.
Witch is a ranged champion who can be played as either a Top or Support.
As a Top laner, Witch is fast and mobile due to both her passive and Cull - her dash (E). She is very squishy, with low base health and defensive stats, but has high sustain due to her use of Clarity stacks for either mana (Alacrity) or health (Wilt). Most of her damage comes from her auto attacks due to Alacrity (passive & active), and she's good at chasing down fleeing opponents due to her high passive movement, her Cull dash (E), and her self-applied Alacrity slow stacks (Q active).
Left alone in lane, Witch has strong splitpushing potential due to her high sustain, ability to hit entire waves with Wilt, quick escapes by Culling over walls, and self-Revive ultimate. You don't want to ignore her in lane, or she'll push it out quickly and eat your towers, fleeing when you return. However, being that's she is frail and not especcially good at dueling especcially in the early game (items help her lots in this regard, more later), she doesn't have much kill potential. She's a passive farmer in early game, who becomes a late-game splitpushing and roaming terror (she's Quinn-esque).
Her kill potential is is greatly limited by her attack speed, due to the reliance on Clarity stacks to empower her abilities. She can use 5 Clarity stacks to empower her Cull to a 1.5 second skill-shot stun (which also puts her in melee range, which is dangerous especially if she misses the stun), or she can use her 5 Clarity stacks to empower her auto attacks with Alacrity (Q), but she can't do both in a timely fashion (especcially in early game): so she really has no burst - but her sustained auto attacks can get pretty troubling due to her Alacrity passive bonus on-hit magic damage, and her Alacrity active attack speed steroid + slow.
She's strong versus things like Darius and Illaoi, who may get kited by her early on, but she's countered by assassins who can get on her quickly and burst (Diana, Akali, etc) and other ranged tops (Teemo, Quinn) who can blind her auto attacks: denying both her damage and Clarity stacks.
As a support, Witch has strong auto-attack based poke in lane, she can grant her Alacrity (Q) to her Marksmen to accelerate their attack speed and give their autos a stacking slow, making them very dangerous - while still having 5 Clarity stacks to use her Cull to dash in and land a potentially 1.5 second stun (even on multiple targets if she passes through them!): before laying down a Wilt on her stunned targets that is now guaranteed to hit. If her Marksmen dies, Witch can Revive them - making her a particularly high priority (and squishy) back-line support - who paradoxically needs to stay close enough to build Clarity stacks off her auto attacks: giving her a very high positioning skill-cap.
Build-wise, Witch has a strong incentive toward magic on-hit damage and AS items, such as Nashor's Tooth, Wit's End, and Frost Queens claim. However due to her incentive toward attack speed builds, she can also build AP/AD hybrid items, or even physical on-hit items: anything from Hextech Gunblade to Blade of the Ruined King. This gives her a wide variety of build paths depending on her role and opponent. Against a high health top laner, she may want Blade of the Ruined King, while for pure AP damage she may prefer Nashor's Tooth into even Rabadon's Deathcap (her on-hit magic damage scaling both off Nashor's and her Alacrity passive (Q)).
Alternately, she can try to shore up her fragility with tanky items, such as Spirit Visage (benefitting her Clarity-empowered Wilts (W)), Abyssal Scepter, Rylais, or even Warmog's. Having more health as a toplaner empowers her dueling potential and also increases the health she can potentially self-Revive (R) with if she dies with 5 Clarity stacks while fleeing. As a support, being tankier better enables her to stay near the front line of combat and reliably build more Clarity stacks for her multi-target Cull (E) stuns, and Revive (R) heals.
an old lady?
Champions Amoline would struggle against: 
Champions Amoline would work well against: 
