Many of my favorite games follow this pattern: simple framework, special abilities. After the first few rounds, each player has a few simple unique choices from unequal forces on each turn. This format readily adapts to game expandability.
Framework: Essentially, one action per round. The number of different actions is large, but only a handful are typically appropriate each round.
Special: Each character has a set of special abilities based on stats, class, race, or equipment. This structure directly leads to an infinite amount of classes and class abilities, races, spells, weapons, feats, and skills. And you can pick or choose which to play with, without breaking the core structure of the framework.
Magic: the Gathering
Framework: Untap, Upkeep, Pick, Play, Attack, Play, End (plus Respond)
Special: Special abilities are written on the cards. You can choose to play with or without any of these abilities by including or excluding cards with these abilities. The number of cards continues to grow, of course.
Framework: Settle, Build, Mayor, Craft, Trade, Ship, Prospect
Special: The buildings, each of which grants special abilities during a particular role. Change the game by changing the buildings.
Framework: Flip, Point, Place, Invite, Play, Reveal, Settle
Special: Special cards (edicts, flares, etc) and special powers allow you to break the rules. It's easy to add more powers or special cards to the game.
Framework: Action, Buy
Special: Each card grants special abilities or additional actions or buys. Expansions are already coming out fast and furious.
Framework: A series of steps each round
Special: Each piece usually has unique abilities, such as range, movement, damage, etc. Actually, I don't like playing war games very much, because I'm terrible at them. But I really do admire them.
So naturally, I'm looking to design something that fits this format.