Playing a game with special rules to create a new challenge is a great way to revisit your favorite games. Today many games have trophies or achievements for playing a certain way, but that was not always the case.

What are some rules you have used for playthroughs to make it more interesting, fun or challenging?

Some examples of rules I have used when playing a few games.

Resident Evil 4 - Not using any firearm. There is no secret that I played this game quite a lot. One thing that I really like is to play using only handguns but it is possible, with the exception of two instances to beat the game with only the knife and environment hazards at your disposal. Make for a difficult and frustrating run with a lot of, yes, running.  

Diablo 3 - Only using thorns as damage. A stat mostly seen as useless. Collected all the thorns for my barbarian for a while and then played a few bosses and levels just standing around taking a beating. Not rewarding and frankly not fun, but still.

Fire emblem Blazing Blade - Only playing with females. There are a lot of maps that auto deploy male units, but as long as one strip them of any gear it is ok. Your arsenal will be lacking in knights, warriors and heroes and also not a single thief, but with pegasus knights, healers, archers and even a lone paladin it is still possible to beat the game. Savor that angelic robe, HP will be lacking. 

Sekiro. Been fiddling around with no deflect and only deflect to try and get better at the game, no hit and combinations. I fail a lot but when I do pull it off it feels so cool, I managed a no hit run up to just after Genichiro got shot on the way to the corrupted monk and haven't went back since. I want to fight the final true boss, Isshan sword Saint with in a no hit run but it seems so impossible, the second and third phase are so brutally difficult but it's fun trying.

I like to do no-bomb runs in Shmups. Meaning my goal is to never use a bomb to get myself out of a jam.

Oh and I found a way to make a (partial) Thorns build to work in diablo 4, there's a unique chest piece that can drop for Druids that'll give you an insane boost to thorns, the first I got was 10k thorns damage. Do some fiddling with gear stats, boons etc and it's very effective. Although I never thought about using it as thorns only I can see it actually working to melt enemies..

Master System ice hockey game Slap Shot was too easy, so I started to handicap myself by playing one period in every match using the pad with my toes.
I think the average is that I lose the toe period by one goal, sometimes two or three, sometimes tie or even win.

I've tried to play Wing Commander: Privateer using only the initial ship, but never finished the challenge.