Simple tips for designing champs
Follow the Three P's
Personality
Purpose
Power
Personality
A very important part is having a defined personality that is multifaceted and interesting. Having one dimensional characters is hard to pull off in interesting ways. Putting foils in to help make each part of their personality pop out helps in defining them. Varus, (old) Trundle, and Yorick all have great lores and personality that can help you understand how to do it.
Purpose
As with everybody, they need a goal in life. It could be world domination to a goal of not having a goal. They need a reason to exist in the world and a reason to fight.
Power
This is important when it comes down to designing the skills the champs gonna have. You want it to be defined as well as you can. The more defined it is, the easier it's going to be when it comes down to the skills. This also helps when defining what role you want them in.
Now the best champions are going to have all 3 interact with each other, and the more the better.
Take 
.
He has the first P, with his unending cheer and goodwill to everybody. "Today we fight each other. Tomorrow, we may fight together."
His second P is to protect people.
His third P is his strength and his magic door.
Now how do these all interact with each other? The first P helps grant his second P a good explanation. "My shield is here for you." And then in turn his second P influences what skills his third P is going to be about. He could of been anything related to Russian Strongmen but because of his wish to protect it gives him his role and 2 entire skills.
Examples of champs that follow the 3P's well are: 
Examples of champs that don't : 
