Just like any product, a board game has a lifespan. Whether you plan it or not, there is a limit to how many times you will ever play any given board game after you buy it and that's the lifespan. The goal of a good game designer is to maximize that lifespan as much as possible. I would really prefer you to play my games more than once. So what tools do game designers have to help extend the longevity of their creations? One great tactic is creating board games that are different every time you play them. There are several ways to accomplish that goal.
So many games have a variable setup mechanic that it's hard to choose a few as examples. Atlantis designed by Leo Colovini is just one. When you set up a new game of Atlantis, you place tiles in a single-file path from a starting point to an end. Because the tiles are placed pseudo-randomly, the resulting "game board" is different every time. Settlers of Catan uses this same mechanic to generate a unique landscape for each game. This mechanic is a really simple way to ensure players don't fall into a rhythm of performing the same actions, collecting the same resources, or pursuing the same strategy from one game to the next. You can't always try to collect ore if the layout of the board isn't conducive to it! Magic Maze uses a unique style of the variable setup game mechanic by allowing players to choose the set of tiles that will make up the game board prior to playing.
Another method of creating variability in games is to use event cards to apply pseudo-random rules for the game. Many games will incorporate a deck of cards with special rules or events in them. A new card may be revealed each round to create a new combination of effects that keep players from getting too comfortable. "Seasonal Events" is a common term for this game mechanic and it's quite popular among all styles of games. I'll reference The Quacks of Quedlinburg again for this game mechanic. Each round includes a card that only affects the rules for that round. The game is played in nine rounds, thereby allowing for quite the combination of changing conditions.
Many team/group games like Mafia and Pandemic assign roles to each player that allow them to perform different actions or give them different knowledge. This way the variation is achieved not holistically, but on an individual player basis.
You know what game doesn't have any variation in it? Monopoly.
One of the goals I have in the games I design is to always include some variation each time you play. It keeps the game interesting for much longer and gives just a little bit of uncertainty that keeps you coming back. Rideshare is one of my favorite games that takes a serious approach to the variable setup mechanic. The game board is a literal map of Midtown, New York City. Prospective passengers are placed on random intersections throughout the city as well as traffic jams. The same map of New York City can be a completely different place to navigate with a few items blocking your way. I've played it a million times (not literally), and there is no one destination or objective that is repeatedly better than another because the game is never the same twice. That's what it's all about. Have you seen other game mechanics that accomplish this same goal? Let us know!