I think you're looking at this the wrong way round. Rather than thinking, I'd like a blog, what shall I blog about, I think you need to be asking yourself, what am I interested in, what do I know about, what am I already involved in, and then consider how that might be reshaped into a viable blog.
My sister blogs about politics. She's established and knowledgeable in her field, which is somewhat but not directly related. She's also an experienced writer. I think she's aiming to parlay it into a career change, but whatever happens it elevates her profile in a way that won't do her any harm in her current field either. It fascinates her and she finds it personally satisfying, but it doesn't make her any money as such (though occasionally she gets paid to be a talking head on TV or radio). She makes a new post about once a week but spends a lot of time then promoting that post and interacting with other commentators via traditional and social media about it and debating generally.
I also have a friend who blogs about having a child with a genetic disease. Her knowledge and experiences have been invaluable to others, but again, no money unless it leads to something else one day.
I actually have a blogging idea myself that I'm too
to describe in much detail, but the blog would be intended as a stepping stone to a book or TV programme. It's related to my day job and I feel passionate about sharing it and would do it even if I knew there was nothing in it for me. 
Being honest, I don't think what you're currently considering is viable as an earner unless you (a) write very well and (b) come up with something very original and interesting/inspiring to others. I also think you need to think in terms of what it would be leading to rather than as an end in itself.