I kind of pre-emptively made a plot, and backstory for my villain, written as if an archive entry, as I haven’t actually started the story itself yet. But I love it so far, even if it loses. Depending on the guidelines this year (which Doctors you’re allowed to use), I may not submit it though. If I change the Doctor and that Doctor is excluded this year, the story would lose it’s thematic value. Which is how I know I’ve made something I love 
I submitted last year, like the day before, but the concept was definitely too much for 5k words. So I ended up putting down all my notes into a folder. The folder now contains original mythos, lore, planets, and creatures from the front, and from the back is where I store all my Doctor Who ideas.
I wanted to enter in 2023, with a similar concept to the one I have now, but that was when they only accepted Classic Doctors. I also had a lot of exams, and didn’t have the time to understand how to write any certain Doctor, or know how to properly give them a voice, etc. I still wrote down a basic plot, creatures, lore- but I adapted it into my own canon, separate from Doctor Who. It was just too cool of an idea not to make it something original 
Ngl, I personally found most of these winners pretty dull, or average. Written expertly for the word count, but quite standard stories. My favourite are Last Day at Work, and Free Speech. I rate these both the highest. Feel bad saying that though because we need more writers in the Whoniverse, and there are plenty more official stories by actual Whoniverse writers which have elicited the same boredness from me.
And I don’t think we’ve had a female winner at all-(?). Which is kind of surprising to me and a lil sad. Gonna guess more men submit ideas, in terms of a gender ratio. Not a bad thing ofc.