This is only a concept so far. He's a support champion.
Leonardo is a flamboyant painter.
(Passive) Magical brush: After casting a spell he may trace a line by left clicking and dragging the cursor around. This can be any shape. The starting point of the line must be within 600 range of Leonardo and none of the line can exceed 1200 range of him. He is immobile while drawing (he can still move around before starting to draw it however). Releasing left click will end the line and he will have to cast a spell before drawing another.
After finishing the line it appears momenterally to enemies before becoming invisible. Lines appear to allies all the time once it's finished. Only Leonardo can see the line while he is drawing it.
While ready to draw a line a range indicator is visible at all times on where he must start from.
His spells will cause the line he drew to cause an effect to the area it covers. And consume the line allowing him to draw another.
He may not cast any abilities until he has drawn a line.
Lines remain on the field until he casts a spell or dies.
The longer the line is the less powerful the effect. A line less than 100 will have it's full effect. Every 100' more will cause the effect to diminish by 10% to a minimum of 10% effectiveness.
If a spell is activated and it doesn't hit anything 80% of its mana cost is refunded and its cooldown is reduced to 2 seconds (to let you erase a line which you don't have use for, casting a spell you don't need).
(Q) Hell Brush: The area under the line erupts into flame for 2-5 seconds, dealing magic damage every second to enemies under it. The shorter the line is the more damage it deals. (10-5 second cooldown from when the fire fades)
(W) Soothing Brush: The line glows green for 1 second, healing allies that pass through it (only affects the same ally once) for a base amount plus a portion of their missing health. The base amount is lowered for a longer line. (10 second cooldown from when the line fades).
(E) Captivating brush: The line glows blue for 1 second, lowering the enemy's speed which pass over it for 2-5 seconds. The power of the slow is reduced for a longer line. It snares with the shortest line. (10 second cooldown from when the line fades).
(Ult) Inspirational masterpiece: The line flashes crimson, any allies which its over will receive a buff which increases their damage against enemy champions while decreasing the damage they take from them. This will cause a decrease in effectiveness not based on the length of the line rather than the amount of champions it affects (only affecting one champion will cause full effect, 2 champions will both recieve 10% less of the effect, 3 champions 20% and so on) so if you want to affect your entire team quickly scribble everywhere and cast.
While a support he can also be played as a burst mage, drawing a dot then hitting E to snare an enemy, then drawing another dot over them and hitting Q will cause a lot of damage to them.
Lines which backtrack over themselves don't count as being longer.
Have your team group together, unless the enemies have AoE, for bigger healing potential.
Use the maximum range to judge how long your line is. If you started your line at the very edge you were allowed to (600) and drew a straight line to the maximum range (1200) then your line is 600' long.
You may draw a new line when you cast a spell rather than when the line from the spell fades.