There have been no current mentions (that I am aware of) that allude to championship riven being available any time soon, or ever again for that matter. I understand your pain, as riven is my favorite champion and I very much wish this skin would be available again. Also, due to the fact that they are working on and releasing a new 1350 RP Riven skin, Dragon Blade Riven, Riot will most likely wants to let that be the new dominant Riven skin. Mainly because they need to make a return on the investment and time it took to design and make the skin (it does look pretty great). I agree that it is unfair in a way that people who joined in season 3, like you and me, haven't had chance to purchase a great skin for a great champ, yet games like League, want to reward players who have stayed dedicated from when the game was in it's earlier stages, with something that is unique. A great example is when the harrowing skins were re-released they gave a special Summoner Icon to those who already owned the a harrowing skin.
On a side note, you should have seen Riven in season 2! That was really OP. I think Riven is a great champ, and is fairly balanced right now, unless she gets fed, (like every champion). So, keep on enjoying Riven! She is a great champ, and if you want a good skin go for Redeemed Riven, or Dragon Blade. I own Redeemed and it is really cool, and Dragon Blade looks stunning (W).
