Let's say that you wanted to create a new series of rts games. Although you have little-to-no restrictions on creativity in terms of designing your 1st game, you may experience more restrictions in terms of creativity when designing the next sequel. How much depends on what kind of path you'd like to take your series in.
Which brings us to our question: how would you handle sequels to your rts series?
#1: One setting to focus on (aka the path most game series take)
By focusing on a single setting, you are able to expand upon that single setting quickly with each new game. These kinds of settings would no doubt contain one or more memorable characters that you can expect to see within future games in the series. However, the main drawback for following this path is that each game is expected to follow the logic of that series's universe. For example, you wouldn't expect to find magicians within a sci-fi world, or giant killer robots in a fantasy world. However, if your series's universe's logic is flexible, like in the Mario series, which allows for science fiction elements to exist alongside fantasy elements like wizards and magic potions, then you should have fewer limitations towards creativity. The vast majority of game series follow this path, including such series as Mario, Kirby, Halo, Gears of War, and Starcraft.
#2: A different setting for each new game (aka the Final Fantasy path)
By focusing on creating a brand new setting for each new game, you're given almost absolute freedom in terms of creativity. One game in your series may take place within a gritty post-apocalyptic sci-fi wasteland, while another may take place within a fantasy setting, and yet another game may take place within a cheesy retro-futuristic setting. While this does mean that each setting wouldn't be flushed out well, with players not expecting to see the same character appear again in a different game (except as cameos), recurring gameplay elements, creatures, weapons, names, or even characters are not unheard of. For example, the Final Fantasy series, despite having a different setting for each of its games, has a number of recurring features, such as spells (like Firaga, Esuna, and Reflect), classes (like Black Mages, Blue Mages, Berserkers, and Dark Knights), monsters (like Moogles, Chocobos, Tonberries, and Cactuar), monster summons (like Shiva), items (like Phoenix Downs), an airship, and even character names (like Cid and Garland).
#3: Multiple sub-series within the meta-series (aka the Command & Conquer/Mega Man path)
By focusing on creating multiple sub-series within the main series, you are given more freedom in terms of designing new games than by simply focusing on a single setting. Each new game can fit within either an existing sub-series (which would expand upon that sub-series' setting), or a brand new sub-series created to harbor the new setting. This has the benefit of giving designers more freedom in terms of creativity than merely sticking to a single series, while still allowing designers to expand upon each sub-series's own universe with each game. You can even expect to see a number of memorable recurring characters from one game in the sub-series to the next, or, in some cases, between games within different sub-series within the meta-series (such as Zero, who was a sidekick to X in the Mega Man X series, and the main hero in the Mega Man Zero series). Examples of game series that follow this path include the Command & Conquer series (which is divided up into the Tiberium, Red Alert, and Generals sub-series) and the Mega Man series (which is divided up into at least 7 sub-series, with most, but not all, series taking place at different timelines of eachother).
#4: Retelling the same story over and over and over... (aka the Dynasty Warriors path)
By focusing on retelling the same story over and over again with each new game, you're given little freedom in terms of creativity. The story would usually need to play out the same with each new game, though you may see the story through the eyes of another character, and the game may even allow for branching plots to occur. Dynasty Warriors is the best example that I could think of, as it's based off of the Chinese classical novel Romance of the Three Kingdoms. Games that are based off of episodic television franchises, such as the Dragon Ball franchise are typically like that as well, though some games do offer what if? scenarios.
Personally, I would choose to go down the Final Fantasy path (aka path #2) when designing my own rts series, with the Command & Conquer path (aka path #3) coming in 2nd place. That way, I'll almost always have a lot of freedom in designing new storylines, technologies, factions, characters, etc. when designing a new game for my rts series, though I may keep gameplay mechanics more-or-less intact in-between games, and may even feature recurring technologies, creatures, characters (sometimes as cameos), etc.
Edit: Thanks for the poll guys. I appreciate the help.
#5: Something else.
This is if you'd like to go down a path that's not like the other ones mentioned above. If you vote for this, then please explain what kind of alternate path you'd like to take your rts series in.