Well the obvious would be the beatles, but why not maybe do four (if there are) one on each Beatle's solo work? You know, Lennon one week, McCartney next, then Harrison and ... oh. Well, maybe three.
It's hard to say. Sometimes it's nice to go for something a little unexpected. People knew I was into metal but not the heavier/nastier stuff, which I'm not, so I thought it might be good to do a whole month which would allow me to explore bands I have never heard, see if I could hear what the likes of the Batlord hears when he racks up High on Fire or Morbid Angel (I don't) and also to prove I wasn't afraid to at least listen to bands like Hellhammer, Celtic Frost, Slipknot and Carcass. Now at least I can say I have.
But it would have been much more predictable for me to have done a Prog Rock month (may still do) or even an AOR one, because these are my primary areas of interest. I don't know yours, so really, as a certain Samuel Clemens once said, or to paraphrase him, the best person to make that decision is you. You know the music you're into better than anyone. Maybe you're a closet jazz fan? Into punk? Hardcore electro? Without knowing your tastes it's impossible to know. I saw you did a thing on video game music, but would that stretch for a whole month? A month of solo artists? A month of instrumental music? A month of music from around the world, a different country every day?
The possibilities are endless, but if you do a month rather than a week you'll have to bear two things in mind: one, you will pretty much need to update every day to keep the interest going. Two, you'll need more than album reviews or people will get bored. You need some sort of overarching theme to link the weeks, like I did with The Meat Grinder and to a lesser extent The Metal That Made Me. Oh, and it'll need to be a subject popular enough to again keep the interest, so a month of Norwegian Noseflute Music probably ain't gonna cut it. I know, that's three things: I suck at maths.
Just think about it and see what you decide on. Maybe signpost it a while beforehand as I did, see what if any interest there is. And be prepared if you go for the big one: I researched and reviewed for two months before I let Metal Month hit the journal. There's a lot of work involved, and once you start you're in; there's no turning back!