What is player set difficulty?
Have you ever replayed a game you are familiar with, but instead of just going through normally you imposed some restriction? That could be not using an ability, a mechanic or whole system. The most famous of these self-imposed challenges is the Pokemon Nuzlocke challenge. For a rundown look here. For a Nuzlocke run it is basically imposing rules on what Pokemon you can capture and when a Pokemon faints you have to release it. There are a varying optional rules depending on what challenge you want to give yourself. One of my personal experiences with a custom set challenge is Final Fantasy X. I think Aeon summons make the game way too easy so I set myself the challenge to not using any Aeons. Apart from one boss fight early on where you are pretty much forced to use an Aeon summon, the game was not that difficult to ignore Aeons (apart from Seymour!). The next time I run through it I will do a run where I swap classes and make the usual mage the heavy damage dealer, which is nice that the game allows for that freedom where any character can be built up to any class or a hybrid of classes.
Why do I bring this up? What do you remember more fondly beating a game on the highest difficulty or setting a challenge for yourself and overcoming what you thought could be too much? For me it is the self-imposed challenge. Why? Because it is mine, it is personal. I own it. A lot of people have done the same thing as you when they beat the game on a high difficulty, but how many have done a Pokemon Green (Japanese version) Nuzlocke challenge? This I think also has to do with motivators, something you want to do (intrinsic motivation) is a lot more powerful than an external reward just for the sake of something shiny saying you did good. Isn't what you want is to do leave a lasting impression on the player they will remember fondly. When games are released they become the players game so the experiences should be as well.
This is also a matter of preference. Just like genre is matter of preference but there is a segment of the gaming community that value the self-imposed challenge. For example
The deprived run in dark souls is a famous one (this is inbuilt though). Another example is a 10-minute AFK challenge in a MOBA like Dota 2 and League of Legends (hide in your base for 10 minutes then start. Giving your opponent a 10 minute head start. Example). There is no reason to do this but the feeling of accomplishment for overcoming this can’t be beaten.
How do you design a game where players can set their own difficulty?
I will go through a few of genres and how this might be accomplished:
RPG's are quite easy to allow for players to set there own challenge since experience gathering is the determining factor for difficulty in those game. Obvious example is a level 10 player will breeze through a area designed for the player to be level 5 in a Final Fantasy game. Players can choose to try to tackle challenges they know they will be under powered for. Creating a type of player set challenge.
Pokemon is an easy game so it is natural to create a challenge in order to enhance enjoyment. The variety of Pokemon and abilities allow for this. These Pokemon and abilities are unbalanced (especially in the earlier games), so, do you just create an unbalanced and easily exploitable game? Seems exactly the opposite of what we want to do when designing games. Maybe not but an unbalanced game may not be all bad.
In Final Fantasy X the Aeon system is OP (over powered). So, do you just put in something as OP as that just so you can let people who don’t value challenge breeze through the game and let people who want a challenge decide not to use it for themselves? While having it in there seems to be exactly what we don’t want to as game designers. What we want to do is to create flow and avoid boredom by making it not too easy. So it seems to contradict that. The reason this works is you don’t need to grind when doing a run with Aeons while you may need to if you don’t . I think most importantly the Aeon system can easily be ignored, it is tucked away as a single user ability. People may complain that the game is linear but the combat is not. It allows for freedom of customization and play.
Include a wide variety of weapons so people can do runs using only one weapon. Also, stealth or melee ability so they could try do a no gun run.
Characters in sports games usually have fluctuating abilities so this naturally lends itself to player set challenges. Even making a mode restricting the player is easily doable.
One simple guide for designing for allowing player set difficulty is:
Do not restrict gameplay to one specific way to play. Your story or paths through the game may be linear but how you tackle it doesn't need to be.
It should not be the focus of your game design but something to consider. It is tough with certain type of games like puzzle games for example. However, if you are designing a game of a genre that will allow for this by all means try to include it into the game. Allow for freedom.
This is only scratching the surface of the subject. I hope in a few years time I will have more to say on the matter.