When it comes to creating content for anything, your blog, your Patreon, you may wonder if you need to stay on topic or theme. What if inspiration strikes and you really want to write about something? Should you? Will it hurt?
While topics and themes help your readers know what to expect, they shouldn't be straight jackets. Rather, they can be frameworks. Your readers want to know more about you, which means if you're super excited about a movie that's coming out, feel free to talk about it. Not only may your readers find that they, too, share this interest with you, but you may also educate them about something else to enjoy. It helps provide content which goes above and beyond "buy my book" or "something new about my book".
It may help if you create theme days, or a series of themes, for your project. For example, I tell people that on my Patreon I blog about mental health advocacy and awareness, writing tips, and of course my own work. In this way, those who join get a mixture of things, all of which do relate back to my work and writing in some way.
I think it helps to build a following if people know they can come to you for X topic. However, don't let the fact that something strikes your interest stop you from blogging about it. You may find that it brings more value than you'd expect to your site.