Ok, so there's plenty of debate over this topic, but recently, I've felt particularly touched by it. In a simple, one word answer, No. Riot is a game design company and they're perfectly capable of designing new champions on their own. They may see some interesting ideas on the forums, they may even be inspired by the concepts they see being discussed or the champion's description or pieces of lore we all create for our champions. If you're really lucky, you might even see a champion down the line who incorporates an ability you thought of, or a bit of lore you thought needed exploring or even a champion designed in the same vein as your own.
Or, they just might use your concept entirely, like they did mine ^_^
This my friends is the original concept for a champion I posted way back when everyone was talking about 'female tanks': Tyrdra, the Boreal Damsell. I submitted an idea for a frost giantess from Freljord who sought to unite her scattered tribes by becoming the greatest warrior they had ever known. About a month later, we got Leona and I thought that was pretty cool. She wasn't anything like what I'd thought our first female tank character would be, but I played her and have some friends who still main her. Since then I more or less forgot about my champion, and haven't really felt inspired to do any champion concepts since.
But now, six months later and Riot releases their newest champion to join the league: Sejuani, The Winter's Wrath. A female warrior from Freljord who seeks to unite her scattered tribes by becoming the greatest warrior they've ever known . . . HOW COOL IS THAT?
Sure, she's got different abilities (they're about as different as you can make them and still keep the ice theme for a tank), and she may not be a giant herself, but she's every bit the icy femme fetale with an oversized weapon and bone chilling demeanor I envisioned when I was writing up Tyrdra. Hell, she may not be a Giantess, but she's got a giant boar! Which is every bit as cool I think. I'll even chalk it up to the fact that they've been looking to put in a mounted champion as well, and they've until this point never mentioned frost giants in the lore.
Obviously I don't think some champion designer sat down, saw my post and thought, "Hey that's pretty cool kid, why don't I pay homage to what is obviously a really great concept that you've put a lot of work into." But if my champion even provided the tiniest bit of inspiration for the team that put Sejuani together, call me a happy camper.
In short, thank you Riot, for bringing my champion concept to life (no matter how much actual inspiration she might have provided), and to all you posting your own champion suggestions, keep writing, keep working and keep up the good work.