You've been toiling away on your project for six months or a year, pouring your heart and soul into it and doing everything yourself, and you've made a game that can be completed in... ten minutes.
Hmmm. Sound familiar? A lot of solo developers have encountered this problem, myself included. My desire to have everything in the game be unique and meaningful has needed to be reconciled with wanting to make a game that lasts longer than the average sandwich, and this has been a perennial head-scratcher.
I don't know that I've cracked it, but it's a problem I've been turning over for a while and I have some suggestions...
1. Get more out of your assets
You've worked hard to make your game's assets look and sound great, and it makes sense to try to make them go further. Sometimes this can mean not being paranoid about repetition - your players will probably accept more asset reuse than you might think (although don't push your luck).
It may also mean trying to get players to spend extra time with the more unique assets. If you've designed a gorgeous one-off environment, can you get your users to spend more time in it? You could pop in an extra minigame, non-player character (NPC), boss fight, or secret thing to find to get more mileage out of those art assets.
2. Make it harder
This is a time-honoured indie developer game longevity approach - if your game is so hard that players routinely need ten tries to get past the challenges, the game will obviously take longer to complete.
However! You must not abuse this technique, and players who perceive that your game is being unfair, or just 'impossibly hard for the sake of it' are very likely to get turned off and play something else. High difficulty should never be at the expense of fairness or fun - it's a spice to be used sparingly.
3. Tell a story
If your game doesn't tell a story you could be missing a trick, as adding cutscenes and NPC dialogue can both infuse things with emotional resonance and increase the time it takes to experience your creation. Cutscenes, however, are not to be overused - don't make them longer than they need to be, and don't make your player watch a video of something they could be doing.
You can apply the principle of storytelling both to the overall game and to individual levels and set-pieces. If one of your levels seems a bit brief, consider telling a mini-story within it; could you add a character or a mini-quest to give things more life and flesh out the scenario?
4. Replay incentives
Can you add reasons for people to play a level more than once? For example, every level in Jumpjet Rex offers you multiple stars that can be acquired by beating the level in a target time, or without dying.
Alternatively, you could encourage players to return to hunt out all the secrets - I ended up replaying a lot of the levels in Donkey Kong Country Tropical Freeze because I wanted to get all the K-O-N-G letters and unlock extra levels. I'm not generally a completionist, and I mostly like to do the minimum work to see a game through, but the level design in Tropical Freeze was so good that I could be persuaded to play levels multiple times in order to unlock more.
Another time-honoured approach is multiple endings. You could go the old-skool route of having one ending cutscene for any% completion and another for 100%, or you might go the storytelling route of narratively different endings based on your actions earlier in the game (à la Mass Effect and Undertale) - some players would probably feel compelled to go through the quest another time to see if they could unlock a more agreeable or at least different ending.
5. Add puzzles
How can you get players to spend minutes at a time with the same assets without cutscene padding or repetitive combat events?
Puzzles are a tricky one, because it's hard to get the balance right. You want players to get stuck, temporarily, but it has to be fair and logical. The art of good videogame puzzle design is too huge a subject for this one post, but if you think you've got a good grasp of how to design a great stumper, a few puzzles can work wonders for stopping players from racing through the content. Why d'you think 'puzzle platformer' is such an over-used indie game genre?
However, puzzles are poison for replayability - you can't really ask your players to do the main quest two or three times if they're mostly going to be re-doing puzzles for which they already worked out the solutions. That's not fun gameplay, and it brings us on to...
6. Avoiding busywork like the plague
I don't want anybody to think I'm advocating padding your game with meaningless, boring tasks for your players. Gamers can tell when their time isn't being spent respectfully, and asking them to do redundant or unimaginative activities just to stretch the material out is (a) bad game design, and (b) rude, frankly.
I don't especially want to throw a particular game under the bus, but I will mention this example because I replayed it very recently and it's fresh in my head - this room from Donkey Kong 64:
You first encounter this room as Donkey Kong, but you can't operate the switches to gain access unless you return as Diddy Kong. Once inside, there are further switches for Lanky and Tiny Kong to operate separately - necessitating four visits to this one room to get everything. It's not really a puzzle or a challenge; just stupid, pointless padding.
In the end, though, you shouldn't assume that everyone will play the way you do. You know your own game inside and out, and even though you can beat the whole thing in twenty minutes a new player might take an hour to do the same thing.
In general, too, I think it's a mistake to worry about game length too much. The game industry has what I call Long Game Fetishism, whereby we sort of expect a decent game to eat up 40-60 hours of our time - I think this is bogus, honestly (expect a future blog post getting deep into this issue). I think there is very much a place in the ecosystem for beautiful games of five hours or one hour or ten minutes.
I'd much rather play an incredibly good and meaningful ten minute experience than a lukewarm sixty hour timewaster, and I have a funny feeling I may not be the only one.