game design ideas

I like the Star Wars Lego’s game because the story plays out according to the movies in some places, but there’s still enough room to maneuver within the game to be interesting. I’m reading a fantasy novel with an intricate plot, so I thought I’d base a game on the storyline from my book.
This would necessarily be an rpg, quest game for explorers, achievers and killers.
An ordinary girl discovers she’s granted powers of a witch, and she’s the sole salvation of her people from the evil Gul’gotha. She gets her power from sunlight: just by raising her hand to the sun and simply wishing for the power, her hand will vanish, and when she lowers her hand, it is full of the energy she needs. The magic resides in her blood, so she needs to nick her finger to release the magic. She can’t renew her powers until she’s completely drained of energy, and the more she uses her magic, the more it takes over her. The magic is a tool, but if overused, she becomes the tool because she can’t keep it in check.
Her powers come from the sun, but the bad guys get power from the night. She’s just one person, so they attack en masse at night usually. She has a group of fellow travelers who protect her and who also have ‘side quests’ that are integral to the story: they have to leave the main group, accomplish their goals, and return to the group to add their new strength/knowledge to the main goal. Each character has a small ability to manipulate a certain amount of natural elements (they’re called elementals: one controls the wind, one is a healer, one ‘speaks’ to rocks, one is a shape-shifter, etc.) and their power is not limited to the sunlight, but they tire easily and take longer to be able to use their power again.