A tip for anyone doing a champion concept
Ask yourself "Would I like to play against this champ or be forced to ban it 100% of the time?" Ask yourself "What would I do if I had to face this champ to win/survive if the other guy plays him perfectly? Is there any way this champ could be beaten?" Try to figure things out from the other guy's perspective.
If you insist on adding numbers to your champion's kit, try imagining laning against him if he tried to run certain builds. What would that champ be like at 40% CDR? What would happen if he went full tank or glass cannon? What would his numbers be compared to one of the guys who had to go up against him. What would his numbers be compared to a champ in a similar role? How does your champ's damage or CC compare to a champ you hate laning against or you think is broken?
Does your champ have a stronger poke than a pre-rework Nidalee with the same build? If so, you've probably screwed something up. Is his key skill more annoying to play against than a teemo shroom? If so, you've probably screwed something up. How do his powers scale? If someone maxed one skill first, what would he look like at level 7-8 or whatever with that maxed skill and maybe a rushed item? Would that be any fun to play against at that level?
I'm working on a concept myself (a midlane battlemage based on pre-nerf soraka) and one thing I've been trying to keep aware of is that I'd want to be able to play that champ as much as possible (ie not a pick/ban champ) and I wouldn't want to dread playing against that champ if someone else picked him. While this is making the idea more difficult for me to finish so I can post it, it's also forcing me to make a better idea. It's also making me appreciate some of the things riot is probably going through to make or rework champs.