Hey there!!! I'm actually bussy with a mount Targon Marksman aswel....basically because of the same reasons XD I figured that we needed a champ from that region for each role...
Anyway, what I find the most important in a champion concept is how it ties in with the existing lore, both in kit and in character. It's kit should fit with the champion...It should make sense for the champion to be able to do certain things (like Ahri and Rakan charming people, fits their character and their lore). This will definitely be half the work on the champion. It also offers a great starting point for coming up with abilities. Ask yourself, what would this character do in this situation? What would he/she have on hand?
As for the kit, don't worry, no-one gets it exactly right on the first try. I generally post my idea on the boards by the time im happy with how the kit and the character resonate with each other and then use feedback from the community to get the balancing correct :)
Give me a heads up when you round out your idea!!! I'd love to see your take on a Targonian Marksman!! My current working title is "Aspect of the Stargazer" ^^