Whew, after my first real, proper champion concept, I've realised it takes a lot of work to not only build up a solid idea, but to flesh it out and make a whole package to go along with it. That and balancing. I'll never smack-talk Riot about their balancing choices ever again :T
There are a handful of concepts that are sitting around on my computer right now. About two of them are recreations of characters, which'll need a full lore/visual rework to make them feel less like carbon copies and more like champions. At least they have fun(ish sounding) kits (which both require boatloads of balancing).
The closest one to completion is Komachi, but she's encountered some balance and meta (i.e. role) issues. She's been benched until I can figure out which direction to take her kit and lore, but her personality and general character idea is pretty much done.
The next closest one was one I made eons ago but never wrote down. The idea popped back into my mind a bit ago, thanks to a couple new champions that re-sparked my interest in the concept and give me a base to work on with the design. The kit feels fun to me so far, but I'll have to work on the whole concept and look back on the kit with a fresh mind to really see if the idea is worthwhile to complete.
Finally, let me just say how helpful it is to have a In-League base to help with a champion concept. Sakuya never would have come to life if it wasn't for Ao Shin and Yasuo. Likewise, my current design plays off a trio of champs that never would have passed the 'imagination' phase if they were never released.
A great idea is hard to come by. Someone else's great idea is easy to find, but it'll never feel like yours. But a great idea that comes from someone else's idea works gets the best of both worlds.