No not studied accountancy but supervise Dissertations (both UG and PG) so can advise on the general stuff.
It's crucial to find out about the way that accountancy dissertations are typically formatted - in humanities there is a standard way - what's typical for accountancy?.
As for subjects, you should get advice on this - what are acceptable areas of study - but you will also need to formulate a research question - what do you want to find out about, how will you do it, what do you expect to find out (and then after you have completed your research) what did you find out.
It is a lot of work but then again the module will likely be worth 40 or 60 credits, so that's to be expected.
You can spread the workload out if you start working pretty sharply on it and work steadly across the trimester.
What subject areas have been signalled as suitable so far?