Currently, the broadest possible categorization is "East Blue" and "Grand Line".
I don't consider "Grand Line" a saga, because it has pieces that are too significant (some are the size of "East Blue"), but "East Blue" doesn't have many sub-categories that make sense.. arcs are connected, but linearly, and an arc usually has nothing to do with what will happen a few arcs from now. (Loguetown is a possible exception).
So "East Blue" seems like its the first saga, because its the real tie that binds all of those arcs.
From there, I think there are 3 more sagas: "Baroque Works", "Skypeia", and a third and harder to name saga which I think I want to call "Water 7". These are from entry in grand line to the end of Alabasta, then from leaving Alabasta through Skypeia, and from there to present (long ring presents a difficulty in naming the Water 7 arc as I'll explain).
My reasons for this nomenclature is that it is the main tie that binds together the arcs within those sagas from beginning to end should be the name of the saga. For Baroque Works, we learn about the BW before we learn of the ordeal at Alabasta (During entry (it can include Laboon because of Mr. 9 and such) and also Whiskey peak).
From the beginning of Skypeia saga (the falling ship and meeting the monkey divers), sky island is the major focus. This arc would contain that mini story about the monkey pirates, the jaya arc, and everything in skypeia propper (some people like splitting up upper yard and some other stuff, i don't think it matters too much).
From there its tougher. If you want to lump long ring long land into the next saga, then it can't be called CP9, because there certainly isn't any evidence of CP9 until the crew is well into Water 7, so calling it CP9 potentially ignores the first part of the saga. If you count aokiji's warning, that's still after Foxy. The problem is there is no mention of Water 7 either (that i can remember).
I still have a problem calling this saga CP9; it just feels that CP9 made itself known too late in the arc to take control of it.
note – when long ring raws were coming out (all that time ago T_T;), I was concerned that oda would use that arc to get rid of a crew member for some time, perhaps to be regained at a later date. They make it clear that losing crewmembers can happen in the DBF, both by the ship they see and by the rules Foxy explains. This of course happened (to chopper), but his absense wasn't very long. However, the thought that the crew wasn't one unit but could be split apart had its "official" (canon) genesis here. Since the events in water 7 surrounding Usopp, Robin, and Going Merry, I've come to regard the foxy arc's main function as a way to plant this idea into peoples heads somewhat subconciously.