When a game is part of a series, it can make things easier for both the gamer and the designer. The gamer doesn't need to sit down with a whole new rulebook and read it start to finish every single time she wants to learn a new game in the series; once she's done it the first time, it's just about learning the new rules specific to that scenario or volume. And the designer doesn't need to write a whole new rulebook, and I gotta tell you, the less time I have to spend typing in "Movement Points cannot be shared between units or stored from turn to turn", the better.
Beyond the reduction in manual labor, however, the great thing about doing a new game in a series (or, failing that, building a new game using mechanisms and systems from another) is that the foundations are sound. The major problems were solved the first time around. I know what works and what doesn't - most of the time, anyway. I still have a distressing habit of trying an idea that not only doesn't work, but in retrospect, it should have been obvious. There's a reason, for example, why infantry units in the Shot & Shell series max out at three step counters and not four, but that didn't stop me from trying it in an early version of The Heights of Alma. But as I've said more than once, my process is a lot less about mathematical precision and analysis and more about what feels intuitive, which naturally lends itself to a lot of trial and error. Still, however, that amount of trial and error is greatly reduced when I'm doing the second or third or fourth game in a series.
At the same time, I don't want to say that doing a series game or a sequel is as simple as making a new map or orbat, because I strongly believe that each entry should have its own specific focus or lens. For example, the Shot & Shell rules are really built around the Activation Track, which models all sorts of differences in morale, leadership, and exhaustion. And another focal point for the series is the importance and devastating power of well-positioned defensive artillery.
So every title in the series should, of necessity, work within that framework and those points of emphasis. A battle where those things don't matter - and, okay, I can't really think of a battle where morale, leadership, and exhaustion didn't play a key role, but bear with me here - a battle where those things don't matter is not really a great fit for the series. But beyond that, there is the question of what each specific game brings to the table, and how it is distinctive from the others. For example, the first game in the series, Seven Pines; or, Fair Oaks, is centered on the feasibility of the Confederate attack plan. The second game, next year's The Heights of Alma, is going to get its mileage out of the differences in discipline and quality between the three armies, as well as the definitive advantage provided by the long-range infantry rifle.
Each game needs to have that something - even if it's a subtle something - that gives it a reason to exist, and gives you a reason to buy it and put it on your table. It also gives this schmuck a reason to design it! Finding that special something, giving it the proper emphasis, finding a way to view the entire conflict through that lens - that's not only the fun part of game design, but I dare say it's the most important part.
I've seen plenty of games where folks get the hex scale right and the orbat and the mechanisms are fine, but there's no oomph, no personality, not an ounce of juice. The designer may have gotten the particulars right, but the whole thing is completely lifeless because while they've done the research and the work, they haven't found the heart of the thing. This of course goes for one-and-dones and for series games alike, though I think in the latter it's perhaps more crucial.