I apologize in advance. I started typing this two minutes after the original post, and
just now finished. Needless to say, this is going to be a
very long post, as this is a very serious question that has a lot of considerations.
Whether or not to split your game up into multiple segments is not a decision to be taken lightly. Speaking from experience, this is something I've wrestled with myself. Hopefully the research and thought process I went through can help benefit you in making your decision. There will be a lot of questions you need to ask yourself honestly as a developer, not least of which is how ambitious the story you're trying to tell is.
Take the Xenosaga series, as an example. The story presented in the series is so layered and complex that they had originally planned for it to span
six (!) games. The series was cut short after the third game, the story compressed greatly to try and give fans a sense of closure. The project was so ambitious that it was ultimately an impossible goal.
So, you first need to ask yourself a few very important questions.
How serious am I about this project?: ShowHide
This is, hands down, the most important question you need to ask yourself. How much do you want for this game to happen? If you don't have the drive to see your project through from conception to completion, you're better off stopping here.
Speaking personally, the solo project I'm working on right now is extremely ambitious, in terms of what can be realistically achieved with a single dedicated developer. However, it is something I have been working on in at least some capacity for years. I want more than anything to see it become a reality, so I'm willing to commit as much time as it takes to see it become a finished, professional quality product.
If you're just looking for something to do because you're bored, you're probably not going to have the drive necessary to build a game that spans several installments and is also worth playing. You need to want it.
Am I trying to tell one very long story, or several much smaller stories?: ShowHide
It's a very simple question, but both answers lead to totally different development considerations. If you're telling a long, overarching story that is very complex, you may be tempted to split the game into smaller chunks. Similarly, if you're telling an "episodic" story, the individual plots will have a significant disconnect and may be better off as separate games.
Can the story I'm trying to tell be conveyed properly in a single game?: ShowHide If the answer to this is yes, you might be better off with a single game. Episodic gaming is a relatively new thing: it allows players to experience smaller chunks of the game's story and content at a smaller pricepoint, helping to fund future installments and cutting down on development time.
Unless you are releasing a commercial game, this is not really a consideration. As a small team, it will likely take you just as long (or longer) to make three games of 20 hour length as it will to make a single 60 one, and you risk making the experience disjointed.On the opposite end of the spectrum, if your game is pushing 60 hours of storyline gameplay (not including sidequests, etc.) then splitting it into two games may make it more manageable for players.
Can I split the story across multiple games and have each one be interesting?: ShowHide
This is just as important a consideration as the first issue. Properly pacing a story of any significant length so as to constantly remain interesting can be a difficult challenge. Splitting that story up into multiple segments that each need to be equally engaging can be even more difficult. Do they each have a distinct, beginning, middle and end with suitable climax and build-up points, and can they be tied together well?
If your story does not lend itself to being separated into multiple games, you will be facing an uphill struggle throughout development.
Will this impact my game's fundamental gameplay? Will I use the same engine for each?: ShowHide
This is perhaps the most important issue, but will probably be your last consideration. If you've determined the story can be effectively split up into multiple games you then need to ask yourself... how can I keep the game fun? In some cases, repetition of the same fundamental gameplay mechanics can, itself, be fun as long as you keep adding content. The Pokemon series is a prime example of this. The fundamental gameplay is virtually unchanged after 10 years and it's still fun to play.
Games like this are the exception to the rule, however. How will you go about making each game a fun experience without having players "burn out" on too much of a good thing? Will you use an entirely different engine for each game, or just make subtle modifications to each one based on player feedback?
You need to figure this out before you start production on the first game.
What kind of development resources do I have available?: ShowHide
Last but not least, what kind of resources do you have? Are you a one-man development team, or do you have a team of dedicated specialists at your disposal who are willing to work with you through multiple games? How much combined experience do you have? How much free time? Are you the kind of person (or team) that jumps from one project to another very easily, or do you start something and see it through no matter how long it takes?
I'm speaking close to the heart on this one. The DSC project I'm currently working on was originally written as a trilogy, and has a story spanning thousands of years. The basic framework of the story itself was written over the course of seven years. However, based on the level of control that I want over the game (and my inconsistent availability for development,) I determined it was best pursued as a solo project. Realistically, this means there is no way I can finish the entire story arc in any kind of appreciable time-frame and also have the level of professional polish that I want.
As such, I focused on what I could control. The three major parts of the story each have a distinctive beginning, middle and end. As such, I am concentrating all of my efforts on releasing the first part of that story as a stand-alone game. This way, if I decide to continue development in the future (with, say, a team,) I leave the possibility open without giving players an unsatisfactory conclusion to the story.
I know this has been a very long post, and I appreciate you taking the time to read it. Hopefully this will have given you a few things to think about as you make your decision.
Best of luck with your project.
Edit: ...some editing/formatting. Added another question.