Some very basic questions: When you were brainstorming your beginning and thinking about your character, what did you identify as your character's desire/want/need? What does your protagonist most need in order to grow? Who or what is standing in the way of them obtaining it, and as the story progresses, how will the come into contact or conflict with the force preventing them from getting what they need? In the end, do they succeed or fail to get it? How do they grow or change because of it?

I will never pass up an opportunity to recommend this video: Endings: The Good, the Bad, and the Insanely Great  . It's an hour and a half long, it's made by a respected filmmaker, and it makes some absolutely excellent points regarding themes/philosophical stakes as they relate to storytelling and to crafting excellent endings.

I highly recommend "what if" brainstorming! Toss out some wild ideas for the ending and see which ones feel interesting to you. Once you land on one that feels interesting, run with it and see if you can add more detail. Off-the-cuff example: What if the bad guy wins one of the main characters over to his side? Ooooh yes. What if it's the main character's daughter? What if the main character has to face off against her own daughter in the climax? What if the ideological differences between the two of them are reflected in the world around them and/or the villain's motivations somehow? Alas, those are all for a completely imaginary story, but I love what-if brainstorming and highly recommend it for idea-making, whether you're outlining or stuck mid-draft.