Here's my own suggestion: It's a bit unbalanced, but could easily be modified.
: Marksman, Mage.
(Acidic Shred) Passive:
's abilities apply stacks of "Enzymes" for 5 sec. Each stack shreds 1%, 3%, 5%, armor and magic resist at levels 1, 9, 15. (Max stacks 10) If
damages an enemy champion with another ability, the stack cooldown resets. Also, if
basic attacks an enemy champion with max stacks, he will deal 50,100,150 magic damage. (Stacks are kept, but cooldown doesn't reset)
(Living Artillery) Q: CD 10,8,6,4,2 sec. Mana 40
shoots a long range shot at a small area dealing 20/40/60/80/100 (0.5 AP) (0.6 AD) magic damage. The shot has a 800/900/1000/1100/1200 unit cast range. If the target hit is an enemy champion, it applies 1/1/2/2/3 stacks of "Enzymes".
(Regurgitate) W: CD 10 sec. Mana 40/60/80/100
spews out stomach acid in a small line dealing 30/60/90/120/180 (0.75 AP) (1 AD) magic damage. The line goes through minions but not through terrain, or enemy champions. Adds 3 stacks of "Enzymes".
(Acid Trip) E : CD 16 sec. Mana 60/80/100/120/140
launches a large ball of stomach acid dealing 50/110/180/240/300 (0.5 AP) magic damage initially, then leaves a trail of acidic bubbles for 4 sec. The trail slows enemy champions by 10/14/20/24/30%, and applies 1 stack of "Enzymes" every second. If
stands in the trail his movement and attack speed increase by 5/10/15/20/25%.
(Mouth of the Abyss) R: CD 100, 60, 40 sec. Mana 120
Passive: When {{Champion:96}} dies, he gains 50% movement speed for 3 seconds and deals 250/500/750 true damage in a area afterward.
Active:
gains 100/160/210 bonus attack range on his basic attacks, and abilities.
's basic attacks add 1/2/3 stacks and reset the cool down of "Enzymes". Lasts 6/10/14 Seconds.
Strengths: High DPS, Anti-tank, Large Range, Low CDs Weaknesses: Squishy, Large Mana Costs, Anti-tank, Low Mobility.
Recommended stats: Attack Speed, Mana, on-hit, cool down reduction.
Recommended items on hit:

Recommended items AP:

Extra items:

How to use: Stack up your passive with Q, and farm minions with Q. use W to add stacks in the early game and deal high damage late game. His E can be used to kite, to keep the enemy in range, to improve chance of escape, or simply used for its bonus attack speed. His ultimate a strong game changer, and is very quick to activate. Its cool down period is large, so use it at the right moment or the enemy can flee and engage when it runs out.