This is more an idea about a style of hero that could add to the game as opposed to anything specific but I will illustrate the idea with a sample implementation.
The thought is for a character that doesn't really focus on short term things but more on building up to a game ending coup-de-gras. In this implementation it would also introduce different strategies and desires than the typical gank them as many times as possible, thus making the overall game more interesting.
Fairly weak stats overall, meele attack range.
Q - Parasite
Infects the target with a parasite that steals the following until the target dies:
3% of max hp
3% attack speed
2% movement speed
Stacks up to 5 times and is removed by resurrections like anivia's egg, zilean's ult, etc but is only supressed (not removed) by thing like morgana's shield and alistar's ult.
Leveling it reduces the cooldown from 20 seconds to 10 seconds
Range of 600-800
This allows him to build up and become very strong, but only until the targets with parasites die. This makes him more suited to situations where a team is fighting on their back or where the pace of the game is a bit slower.
W - Leech's blessing
Friendlies near the leech gain 25% of the parasite benefits.
E - Scurry
The leech becomes immune to slows, stuns, and roots for 3 seconds but cannot attack during this time.
Cooldown of 8 seconds.
One of the main reasons for this ability is to allow him to effectively skirmish and apply his parasites.
R - Nightmare
Doubles the effect of all parasites for 10 seconds. After the 10 seconds the parasites are gone.
This ult is intended to be very powerful but the loss of all your parasites makes the cost high so you want to use it when you think it will make a huge difference in the game or to finish off a mostly eliminated team but probably not otherwise.
The leech's attacks drain 1% of the target's current health per second for 10 seconds. (doesn't work on dragon or baron)
This is primarally intended to help him hold his own in the lane and give tanks a little something to think about before they go skirmish with him.
So the overall idea with this character is not so much to get individual kills but to set up game-changing multi-kills and also to prevent single opposing characters from getting out of control.