It's been asked for a long time, but there is no shame in making more animal characters. Volibear and Warwick are too fun to play, a Lion who could uses his claws in melee combat would be just amazing.
Depends how you want it to be implemented. Generally just "lion" is too simple to get implemented in the game and it's lore.
Lion vastaya is unlikely, since it would clash a bit much with Rengar thematically. Best bet in my opinion would be a lion based ascended, the picture you posted here would fit that, too. However, there already has been a lion based ascended in the lore, Setaka. And as far as we know, she is dead.
I don't know if they would be willing to give the same niche to a new champion, I kinda doubt it tbh.
Rengar???? 
He literally has claws in his right hand and is brutal as hell what do you need. A roar? He has that to. Give him a try I recommend it. He even has that assert your dominance and makes you feel scared in the jg alone if hes fed.
