If you've looked at the 'indie game dev' article here, great. That's more about general development strategy - the practical advice for indie game devs. This page, however, covers more specific tips relting to game design.
#1) Scaling. There's a thing called a difficulty curve. Start 'easy' and get gradually harder as the player progresses. This may mean stronger enemies, trickier puzzles, etc. Don't make things too hard immediately or you will alienate players. But it's just as bad a mistake to remove challenge as most players crave challenges.
#2) Players play games for many reasons, but academics studying the field note four major types of players. Social players like multiplayer and gaming as a medium for relationships and social interactions. Explorers, like uncovering the game's world and its storylines, including hidden details and secret areas / easter eggs. Competitive players are focused on games as a challenge and are often focused on beating an opponent. And acheivers, or completionists, try to finish the game 100%, completing achievements. And I personally would add to these the creative player or modder, who builds things into the game setting and uses the game framework as a medium for creating things. The more of these motivations a game can appeal to, the wider the potential market for that game. Just remember that the aim to manage 'mass market success' often results in feature creep, which is a problem for indies with minimal resources! You probably cannot do everything. Choose what audience segments are important to you and prioritize those, while also trying to appeal to others if there's a way to do so without drastic increase in development time and cost.
#3) Length. I personally am not a huge fan of grinding. I've seen it everywhere and as a player who has a backlog of unfinished games on Steam... it drives me nuts. There's no excuse for dragging out a game with content that is clearly repetitive filler. Try as a designer to give every location, every character, every mission a unique element so it won't all feel the same. Speaking of grinding, I am not thrilled with the F2P or 'freemium' trend as it often leads to foundationally broken game design. I would advise freemium designers to focus on customization or cool extras as in-app purchases but to avoid 'pay to win' design that slows progress for non-paying players to a crawl, and rewards those who spend ridiculous sums of money. I may not be offering great business advice in this, but I personally believe in low pricing on everything, even at the cost of lost profit or no profit. For me, I would rather have happy players feeling they got a good value, than angry players who resent me. Moreover, I do have a strong conviction that other things matter more than money. Freedom matters, life matters. People matter. I find it appalling that we've essentially built - as a society - a cult of capitalism above all else, above health and human well-being, above the natural world, above any sort of compassion for others. This upsets me and I don't believe there is any easy solution available. I might stand staunchly in favor of the left wing, except that the concentration of power in government is dangerous to freedom, as any government strong enough to steer public behavior in favor of economic justice and a sustainable [ie not so environmentally destructive] system, is inherently also powerful enough to restrict basic civil rights. I'd like to believe that humans can be influenced towards empathy and generosity without centralized government coercion, through avenues such as charitable giving, but that may be a fantasy. If I can influence you to take this moment to give to a worthy cause, though, that'd be great, I'd love it if you chose a charitable cause you believe is doing worthwhile and effective workwith clear, tangible results, and support them. And I'll do the same every now and then because I believe that it's important to help others.
#4) Social responsibility and themes. I stated it earlier and I'll reinforce it again. Games are a means of self-expression but that expression ought to be subtext, not in the forefront of the design in such a way that it interferes with play or grinds a narrative to a halt. Try to treat players with some respect. This is discussed further in the 'writing' section, how to make show, not tell really work.
Basically, you've got to acknowledge the potential validity of the opposing viewpoint, on your theme, and make it a credible alternative such that the villain is doing what they believe is best and not just being evil for no reason. And you express theme through actions and reactions, and the fallout of them instead of just verbally preaching through a character's dialogue. The theme is baked into the structure of a story, then, and is not just a tacked-on monologue.
#5) Risk taking. You can and should push for a few key innovations or fresh ideas in any project, especially indie. Indies can experiment, and should, so that is a consideration. Prioritize where to innovate and where to go with a setup that's straightforward. Often a good design takes creative risks in specific places, while avoiding the pitfall of being so out there, that it lacks any adherence to good design practices. Also, don't be paranoid about what people will think - just do what works best, and accept that it will not make everybody happy. You cannot make every player happy, people will always find things to complain about and those criticisms can be useful input in future projects, or in some cases too vague, or too cruel and nasty, to be useful, attacking you with curse words instead of offering valid and specific criticism of your work. But there will be criticism, and some of it will come from mean-spirited jerks. Try to separate the useful, from the useless, criticism and ignore the deranged haters.
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