DumasStark

If you're still not sure about the story, perhaps I could provide some ideas.
Could be that your OC is the oldest of the Elza sisters and she looks for treasures in order to get money (and because she likes it. ). They were affected by a curse that they accidentally (and seemingly) put onto themselves as they were practicing witchery. It transformed them into familiars that represented their souls, so the fact that she's rabbit-like would make the fact that she's super quick full of sense. You could make it so that some treasures that you get and successfully research gives out a certain upgrade, like more health, speed, defense, or even abilities, like a double jump by allowing your ears to become bigger at the moment of the jump, but with the only limit that you could carry three of those upgrades with you at the same time, unless you bought better equipment (which would be quite expensive and you would pay by selling the treasures you find, some of them only worth money and not magical properties).
Another thing would be adding your sisters to the gameplay, which would be interesting. I was thinking that the second oldest would be a very studious one, allowing you to research the treasures and thus giving you more stat bonuses, at the beginning you were just able to equip something and notice that they were magical, giving you like a third of its stat properties and not any extra ability. She would try to stand out in her form as the most intelligent amongst other witches, trying to prove her worth like that despite her form. The third sister would be more of a brawler, who could teach you techniques that would help against enemies (like some sort of life-drain between others), and she would be the one that tried to stand out by being the strongest amongst other witches despite her form, be it magically or physically.
The youngest sister would be the one that could set the story and the reunion of the Elza sisters (since they could have split apart or they were simply doing their lives, since they had been in their current forms for years now). She could have been the one with the most magical potential from her sisters and could be the "damsel in distress" in the whole story, being possessed by spirits (that could have had something to do with their familiar transformations in the first place) just before she had become an adult and right after discovering something crucial story-wise due to a dream. She would have been living with the oldest sister, so that she was the one to witness her transformation into a channel of the spirit realm and thus acquiring a more feral and powerful form, fleeing from the city as she caused havoc and mayhem with her out of control powers and was trying to battle the spirits that were trying to take over her. As time went on, the spirits would take more and more control over her, becoming more powerful and feral each time you managed to catch up to her after every couple of stages, also giving you boss fights. Chieko would at first try to stop her sister, but would be brutally defeated after she fights her at the first city where they both live, and would then be forced to look for her other sisters in order to rescue their little sister.
There could be stages that focus on traveling to other cities and have more enemies while others are focused more on treasure hunting and are filled with traps. The first ones could be used to level up more frequently and the others to get more treasures/upgrades (while the first ones could also have hidden paths that could lead to treasure stages or some spots with lesser treasures and a single, difficult to acquire collectible of sorts. The second ones could also have mini-boss fights before getting to rooms with lots of treasures.)

If you're interested I still have more to say, but so far what do you think?