This is an interesting thread.
I would put forward the view it's easier to write for an existing alien, like the Daleks or the Cybermen (either type) or the Borg. The reason - a lot of stuff you can shorthand. Creating a new alien, you HAVE to fill in a lot of background detail, whys, wherefores, evolution, culture, and so on, all of which is time, and therefore script, consuming. OTOH, if you do use an existing alien, you have to get it exactly right, and even then it may be overturned by a later TV script (the sort of thing we see in ST novels all the time, compare the Reeves-Stevens novel 'Federation' to 'First Contact').
But I digress.
I have to ask: do you
only want to write for Who? Is that
all you want to do? In effect that's fanfic, and the Beeb, as far as I can tell, wants to encourage writers who can do much more than that. Indeed, look at the other work of Cornell, Davies, Moffat, et al. They are not one trick ponies. They come up with marketable, interesting ideas for series and movies and adaptations. Which is what they want from new writers as well. Not necessarily because they'll work cheaper ( they might, but I think the Writers' Guild insists on standard rates), but because the Beeb, like an immortal vampire, constantly needs new blood (see if you can get that image out of your head later

). Davies and crew aren't always going to be around, or may grow stale (yes, I can hear the comments now

).
So ask yourself - what writer do you want to be?
I want to write for Who, but I'm on that slow boat. First script I sent was a family story, in the style of Poliakoff's 'Perfect Strangers' but set in Australia, which was rejected outright. The second script is based on a book of short stories (I got the author's permission), and it is going through the process. It's also required a reasonable amount of original input and research, which shows in the script compared to the original stories and the episode loglines.
I'm working on a third script, a murder mystery thing, which is grittier than, say, Marple, but not as gritty as, say, Cracker

. I think there's a market, and it'll allow me to slip in some sly humour and a bunch of other stuff. And I still have to come up with some story ideas to round it out to a series, but I'll get there.
So maybe, around the time of the 12th Doctor, I can begin adding my own stories. Sounds like a plan to me!
