Well, I dunno. Lots of stuff. How big do you want your picture to be? I guess it depends on what kind of book you’re writing.
As I said, I think this book started out as being about Roman citizenship. If you’re writing that book, then you’re including and leaving out stuff for good reasons, because you actually have a theme. Like, to stay with Mary Beard, her book on the Roman triumph. And if citizenship is your theme, then you also have a good reason for ending with the Edict of Caracalla, as opposed to it being just a weird place to stop. But if the book is trying to be, as I saw one review put it, everything that Mary Beard knows about Roman history that she can fit into five hundred pages (which it certainly isn’t, but anyway), or even a general history of the Republic and its transition to the Principate, then it’s different. Then things are being skipped over more at random. For instance, you’ll learn about Gaius Gracchus (I certainly learned stuff, it was great). Or she’ll get into Cicero or Augustus, in a “from scratch” sort of way. On the other hand, Marius is barely mentioned (just to take one example). So, what is the assumption here, Mary Beard? Are you assuming that I already know about Marius? If so, then it’s safe to assume that I know about Cicero and Augustus, too, so why go over that again? But if you’re assuming that I don’t, then why tell me about one and not the other? It becomes arbitrary.
Now, I don’t really want to dump on Mary Beard, especially since the book is actually full of good stuff, and I certainly learned quite a bit (not that I think she cares anyway, as I’m just some layman asshole on a message board, while she’s an expert on the subject with knowledge coming out the wazoo). Other writers in the genre, like Adrian Goldsworthy or Tom Holland, seem to have the same problem (Goldsworthy, especially, has it bad… not that I should really be dumping on him either). It is a real problem. Do you assume that the reader already knows Roman history, in a general sort of way? If so, what you need to talk about is just the specialist stuff, and then who is buying that book? You might as well publish in an academic journal. Or do you assume no prior knowledge at all? In that case, you really should explain everything, and then you’re in a rabbit hole that never ends, and you’ll never even get to what you actually want to talk about. It’s a nut to crack, for sure.
Now, the nut has been cracked once, sort of. If you actually want the big picture, OP, I think (speaking as a reader and a layman only) that you’ll want the above-mentioned History of Rome podcast. Mike Duncan doesn’t discriminate, he’ll just tell you about everything, including stuff that no one else ever goes into, in a no-nonsense sort of way, up to 476 AD. It’s not the whole picture, but it’s a certainly a comprehensive picture, without big holes anywhere. Then you can come back and read your Mary Beard, Adrian Goldsworthy or Tom Holland, and know where things fit in the grand scheme of things.
Really, everyone should check out the History of Rome podcast. Or at least it’s an approach that seems to work for me.
And yes, I know that I’m recommending that every five minutes around here. I’ll keep doing it. And yes, I also know that podcasts are for the lazy, and real men read books. If there was a book that did the same thing, I would recommend that. But I don’t know of any.