I think making an A-Level in something very specific is a bad idea - you can go to colleges for more specific courses, the whole point of A-Levels is that they're broad enough to be taught generally. If there was an A-Level in Architecture, few schools would likely have the resources and teaching capability to actually fund and run the course.
That being said, I'm actually a Scottish student who did comparable-but-admittedly-slightly-different Advanced Highers rather than A-Levels. The density of students and size of schools is much lower here, so maybe down South there'd be places where it'd be easier/more viable to teach more specific courses, though in my mind that's still what colleges are for.
I think a good addition would be Mathematical Physics, which would function similarly to Further Maths - an additional Physics course that builds on what you learn in the regular course, but with a focus on more in-depth and complex mathematics. At the moment, Physics as a subject in schools at A-Level and Physics as a subject at university are very different, with the latter being far more mathematical, to the extent that your Maths A-Level is actually considered more important than your Physics A-Level when applying for a Physics course (or so I gather from some top unis' entry requirements sometimes including A* in Maths but only an A in Physics, though admittedly at most the A* can be in either.) This is apparently an issue, as a lot of students go to uni for Physics and instead get a completely different, much more mathematical subject presented to them, as the A-Level encompasses very little maths.
Alternately, I think a proper Computer Science A-Level would be a good idea. At the moment, the CS A-Level is considered "useful" but completely non-essential by unis when applying for a CS course. If the A-level named after the uni course is considered useless and irrelevant enough to be a nice afterthought in the requirements, with Maths/FMaths and Physics being the actual essential courses, then there's clearly something wrong with the course.
For the humanities side of things, I think the suggestion above of World History/World Politics is an excellent idea. Perhaps an "International Studies" course, or something. There's a huge focus on the UK's history and politics in the current courses, and though that is obviously important, I imagine there are many Politics/History students who'd like to study topics outside of the UK too.