Work in progress. Numbers and abilities are subject to change.
Lunasa, if she was hypothetically released by Riot, is designed to be the final answer to the season 3 assassin meta. Her sustain and considerable array of counterattacks and stuns ensure that she is able to instantly and severely punish anyone who "gets in her face" and tries to burst her down. Many popular mid-lane assassins, like Akali, Katarina, and Zed, would find it extremely difficult if not impossible to lane against her.
A good Lunasa will instantly respond to a melee burst attempt from one of these champions by blasting them away with Impact, then activating Broadhead Barrage to chunk them down with the 10% extra damage from her passive. If her would-be assassin cleanses the stun, Lunasa can fall back on Bound to escape under cover of her tower. An exceptionally mean Lunasa could Flash behind them, Impact them into her tower, then chain them down with Voir Dire for a brutal five-second stun combo while the tower does its job.
However, Lunasa is intended to have very clear-cut and easily recognizable weaknesses. Her base movement speed, at 315, is the absolute slowest in the game. In addition, none of her abilities reach beyond her basic attack range of 600 units. She is very vulnerable to being kited and to ranged harass, and while she is great at punishing opponents for wandering into her attack range, she is poor at actually engaging on the other team. Also, Lunasa relies on her ammo to do damage, meaning that in a teamfight she only has a certain number of shots before she becomes useless.
While she is a strong champion that can absolutely terrorize bot and mid lane, Lunasa needs to have a level of synergy with her team in order to be as effective as other carries.