Regarding solutions to things, my suggestion is to have a tentative solution to a problem and do not, under any circumstances, rely solely on that solution/story thread to move to your next plot beat. It’s important to be flexible and be able to incorporate the emergent fiction of the game into your story.
One thing I usually do when I write out plots is use a mind map. I’ll put the inciting incident of the plot on one side and my tentative eventual goal(s) on the other side of the map, then start filling in possible avenues that could be taken to reach the goals. Next up, I create two sections for players: an active section and a bullpen section. If I know the players fairly well, I can usually anticipate the course of action the players take, which is important for developing potential story beats. Usually I’ll make a note of the characters’ primary abilities or, if I am not staff and have only observed the players, I will make a note of what I’ve seen them do most often. I’ll ask, but people are often really weird about their stats, or they used to be anyhow. Failing either of those, I’ll base it off whatever data I can mine from their wiki pages or stereotypes about those types of characters (e.g., Brujah vampires are likely going to be pro-combat).
Being a (primarily) WoD storyteller, I will also create a three-part approach to stories, with a focus on what will happen if someone attempts to develop a solution to a problem socially, through research (mental skills), or through combat. This will spin out to consequences and forward momentum, which will move to more story beats.
Since this is a largely living document, it also allows me to course correct if things are getting too wildly off path. Something that’s a bit out of left field or way out of bounds of what’s in scope for the plot might be its own spin-off. When that happens, if I can’t run that at the same time I’ll provide the player with a timeline of when we can approach it, then do some redirection if possible. Usually, but not always, those players have followed the plot to their own personal satisfaction at this point and are more interested in this sideline piece. Those players will get shifted down to the bullpen which helps me adjust the weight of some of the other possible outcomes that might come into play given the existing scenes/story beats that I’m going to be running. Do this enough, and you start to see a pattern emerge, and you can usually predict what course the story will take. Despite that, it’s important not to be reliant on your own internal narrative for the story, even if you have a fairly conclusive “big showdown fight at Important MUSH Location #1” ending.
All of that is, of course, for a longer-running thing. For shorter plots or one-shots I generally will have no ending plotted out at all, and will usually just build a few responses based on the approach type of the players: either that three part physical/mental/social, or if I’m very aware of their capabilities and reasonably sure they’re all going to show up, a set of more personalized challenges that the players will overcome based on their strengths and limitations.
This is a lot of work to do and I don’t anticipate that it’s something a lot of people want to do, but I feel like it’s been the path to success for me.