It's way too early to be thinking about this if you're doing a taught masters. Seriously, take a break.
The likelihood is that your thesis will be partially, probably very strongly, informed by content that you cover during the first few modules of your course. Trying to come up with something now is not going to be a sensible use of your time. You will have a supervisor/advisor whose JOB will be to help you come up with a thesis topic. Your uni will probably advise that you want to have a vague idea of what to write about come December/January. Don't stew over your thesis now - make the most of your last few months of freedom, then concentrate on getting oriented to the course, build a social network, get into the habits of academic life.
A piece of advice - work/life balance is really important for a masters course that's largely independent study. Be disciplined with yourself about knowing when is appropriate to work and when is not. I'm just finishing, and I've seen a lot of my peers get into really unhealthy headspaces because they don't know when to/feel like they shouldn't switch off. It's really unnecessary.
Masters courses are generally not particularly difficult. Speaking from experience, if you've been holding down jobs for the last few years you will likely already have the discipline and self-motivation to put all the requisite hours in. If you can manage a 35 hour workweek on average, you'll have no difficulties (might have to be willing to put in some extra hours during peak periods/deadline log jams, but that's OK). Don't go down the road of worrying prematurely before you've even started - with independent study you are your own worst enemy, and psychological hygiene is really important.