It looks like GameswithTwo and I have had some similar ideas for blog posts lately. I kinda started the talk on expansions, not that the idea is original to me. GwT now has 2 parts to their expansion series, so I'm going to do a follow-up to discuss some comments I got here and over on BGG.
Expansions take many forms. I want to take some time to discuss the different types of expansions we see for different games. This is by no means a complete list, nor is it meant to tell you what to do. I'm just trying to offer my own opinions, as always, and you can take them for what their worth, and since they're on the internet and reading this is free, that's about what they're worth. (Trying to inject some humor, just go with it.)
Increased player count with something else
These are probably the best kind of expansions. You take a game that was really good with 2-4 people, and now a 5th player can join in. This has been done in a lot of games, Carcassonne, Cosmic Encounter, Battlestar Galactica, Shadows Over Camelot, just to name a few. More players is typically a good thing, but we have to be careful that the game still works well at that number. Just because a game plays 3-8 players doesn't mean it's a good game to play with 3-8 players. Maybe the sweet spot is at 5 and it works well at 4 and 6, and then ok with 7. More players typically increases the play time of a game, so keep that in mind as well. Maybe that 60 minute game with 4 people is great, but a 5th would make it 90 minutes, and that's just too much for what the game is. That's not always the case, but something to keep in mind.
New "modes" of game play
This can be something as simple as variable map set-up or scenarios for a game you already know, or it could be a completely new way to play the game. Seafarers of Catan did this for Settlers by giving players boats to build and islands to explore. Battlestar Galactica took a page from Arkham Horror when it comes to expansions. The core gameplay is similar, or even the same, but there are new characters, new cards, maybe a new way to end the game that players can pick and chose from.
This type of expansion gives players flexibility as well as new ways to play the game, which kinda makes it feel like a new game each time.
There are several games which utilize new maps to bring in something new to the game. The main three that I can think of are Ticket to Ride, Power Grid, and Formula D/De.
Let me address each one.
Ticket to Ride started by making full games with a new map, so if you wanted a new map you had to buy a $50 game with train pieces and train cards and new tickets. This wasn't horrible, but in 2011, Days of Wonder released 2 Map Expansion Packs with a double sided map. These cost around $20-$25, which is a great deal. At this point I have way more Ticket to Ride than I'll ever get bored of, but that's a good thing. It means every time I play it, I can explore some new aspect of a map, and continue to rotate through different things. I consider the Ticket Map Expansions to be well worth it.
Power Grid is an interesting case study. I enjoy Power Grid, but I play it maybe once a year. At that point, I don't need anything more than the original map for the game. At one point I had 3 different map packs, and I think I played on at least 1 side of all of them. I had a friend who really liked Korea, so we did that map with him, and so on. What I found was that each map did bring something new to the table, but I ended up trading away every map expansion because I didn't have the need for them. I've yet to feel like one of them was crucial to enhancing gameplay, or made me like the game more. Rather, each map just brought something different to the table. If I played Power Grid more often, say once a week or even once a month, I'd probably want a new map set, but right now, I'm content with just the original map.
Formula D is a blend of Ticket to Ride and Power Grid. First, the backstory. My dad and I are long-time fans of Formula 1, so this game is a great fit. It's great to race around all the different tracks that we see on TV. Even so, they don't get used. We play it so little that if Monaco was the only track we had, we wouldn't get bored with it. I love the idea of having all these maps, because we do get new ways to play, and it would be a lot of fun to race a season over the course of weeks/months. The problem is, I just don't see it happening, especially since so many of the classic tracks are long OOP, and rather pricy to acquire.
For the next part of this series, I'm going to talk about 2 games with a lot of expansions, Dominion and Carcassonne. I'm going to look at the advantages and disadvantages of each and every expansion so far, which will be a long, but hopefully useful article. I'll also try to give some advice on where to call your collection complete in terms of those games, but remember, there are no hard and fast rules to games.