Mix and match champion passives and abilities
To make the most broken champion
Passive: 
Perma stealth and mana regen.
Q: 
the weakness of Cho'Gath's Q is that it's easy to dodge when you can expect it.
W: 
stun and gapcloser
E: 
even more stun/peel
R: 
the only weakness, high mana cost, is negated by passive
Can be built full ap assassin or a tank. Incredible roaming potential and dueling potential, as well as splitpushing potential.
Makes him hard to kill, while scaling well with health.
Q: 
Can build pure tank while still having excellent Infinitely scaling damage.
W: 
Is rewarded for playing full tank with more tankiness.
E:
Mobility in spades.
R: 
Excellent last resort for if the going gets really tough.