I think this topic raises the fundamental question of why we write at all, and why we have and need different starting points.
Those of us who want to tell stories often begin with the germ of an idea for one and develop it by populating a setting with appropriate characters to play it out.
Some are more interested in exploring characters and develop those first, which is more likely to lead to what James Ritchie calls 'situational writing' (although that term is more normally used in a different sense, viz. purposeful business writing). In my opinion, this approach tends to lead to the most satisfying novels, because when well-drawn characters are true to themselves in their actions and reactions to the situations writers create for them, (or which the characters create for themselves) they resonate with readers, and stay with them long after they finish the book.
Many writers get their kicks out of plot - taking a workaday story and adding twists to make it more exciting, or new in some way.
Still others are governed to some extent by their chosen genres, which often carry elements of formulaic expectation on the part of readers/editors.
Personally, I am driven by theme, because, being of a slightly didactic and pedagogic nature, I don't want to write just for the sake of it and prefer that my output has at least a point to subtly explore, or make, over and above its (I hope) entertainment value.
(Theme is also one of the principal distinctions between literary and popular fiction - the former has to have it, ((postmodernist self-indulgence excepted)) whereas it is optional in the latter.)
When I have chosen a theme I then think of a story that will be a good vehicle for it, followed by suitable character types. Thereafter I let my characters tell the story, without trying to force the plot, and always have the theme in mind, because it gives the story a basic arrow of direction, a framework in which the seed story can grow, bloom, and cycle back to earth.
I recall reading somewhere (source unknown) about a writer who told her editor that she didn't write with themes in mind. Her editor then pointed out what the themes were in several of her published novels. The best and most successful ones were those that had themes, which she had unconsciously included.
Nirvana for all writers, I believe, is to acquire (through BIC mainly, plus anything else that helps) sufficient innate understanding of story structure that we can write without thinking too much about what we need to do to put it all together. It's like driving a car, but with a wee bit more to take into consideration. It appears to come almost naturally to a lucky few and only by dint of hard work and elapsed time for the rest of us.
I have the morbid suspicion that the only way each of us can emerge onto that sunlit upland it to try a variety of approaches and see which works best.
But I can recommend theme as a starting point, because I find that it does give me an overall sense of where things can/might/should lead, which reduces the tendency to block while still allowing ultimate freedom for my imagination to roam in fulfilling my characters' expectations of themselves.