Brian Ashford wrote a great short essay on rules in OSR games. In his essay he showcased the Thief and the wrench it threw into the game's mechanics.
He talks about how he fixed the problem in his games and makes a good argument that rules should "do more" i.e. encompass the whole game and not just a single class or niche.
I have to agree.
Take ODND style thieves as an example. Their portfolio is comprised of the ability to move with great stealth and climb nearly sheer surfaces. These abilities are fine but what happens when another class decides to attempt these feats? Are they disallowed by the Thief's right of niche protection?
I think most referees have solved this issue by giving each class thief abilities while allowing the actual thief an advantage, as showcased by one of Brian's examples. As you can see these rules introduced from the Thief's inception actually caused more issues and required a more broad encompassing rule to fix it.
So Brian's point that broad and encompassing rules are more efficient than smaller niche rules is a sound enough design philosophy. So let's apply it to another common issue in OSR games: the relevance of the Fighter.
"Fighters are linear and Wizards are quadratic," is a timeless adage that showcases the problem with fighters' relevancy. While the wizard is throwing 3d8 fireballs the Fighter is still swinging his dinky d8 longsword. And as the trend in OSR games to shy away from power escalating items such as +1 weapons grows the fighter's relevancy decreases. So how is this problem fixed?
I believe the popular solution to this problem is to provide the Fighter with multiple attacks per round. There's nothing wrong with this solution but, like the Thief, it brings up a question. If a fighter can attack X times per round how many times can another class attack per round? Surely even my anemic emaciated sleep-deprived wizard is capable of swinging his dagger more than once per round.
Well I'm trying to be broad and encompassing here so I'm going to ignore the finer details like weapon type, physicality, etc and focus on the bigger picture of the roll itself. Here's what I've come up with:
When rolling to attack, if the result is a multiple of seven, you can immediately roll another attack after resolving the first one.
I really like this rule as a base for what I'm trying to accomplish. Translated from ruleset lingo basically this means anybody can attack multiple times if the result is a 7, 14, 21, or 28. (I include 21 and 28 because I make use of a dice chain in my games and therefore the use of a d24 and d30.) It adds a second chance mechanic to attack rolls. Now players don't have to fear the dreaded single digit because there's a chance that zero follows a seven. More importantly this rule is broad and encompassing, which is just what I was looking for, and it makes fighters more relevant with their multiple attacks. But I don't want to stop here; I want to make fighters even more relevant.
It's obvious that fighters desire a high strength score. They hit more consistently and deal more damage. So I'll apply this trend to my new rule:
When rolling to attack, if the result is a multiple of seven, you can immediately roll another attack after resolving the first one. If your character has a Strength score of 13-15 decrease the result required by 1. If their score is 16-17 decrease it by 2. Finally if it's 18 decrease it by 3.
Now fighters of peak human physical condition can attack again on a multiple of 4 and I think that's awesome. Overpowered? Mayhaps but I haven't played around with it enough to be sure.