When you look at a core set of rules for a generic game, you get blanket ideas that can typically be applied to any situation. To make things more flavoured and interesting for diffent characters within the setting, there are often a series of exceptions. As examples, D&D 3/3.5 saw the introduction of feats, where different Characters could purchase different specific abilities that would modify the rules in some way that would be advantageous for them, Vampire: the Masquerade offered different disciplines to different character clans, Apocalypse World (and its ilk) provides "moves" that become available to certain characters at certain times as defined by the narrative.
I'm categorising this entire design mentality as "exception based design", you've got the core rules that "everyone" follows, but everyone has a few specific areas where they are allowed to work outside those rules.
In a core set of rules tightly focused on a specific premise, these exceptions become like chocolate chips or sour jelly pieces in your ice cream. You might get one or two in your spoonful, you might get none...the fun comes in the sporadic nature of their manifestation. They become interesting because they aren't always present.
Then people decide that they want to deviate from the original premise (here's where I get annoyed about lazy the game hacks I see time and time again). D&D 3/3.5 saw a proliferation of expansions and supplements, offering new exceptions to fill the gaps where previously there were none (new feats, new classes which altered the rules in new ways, new tables for new situations), exceptions to the existing exceptions (new ways that other classes could take feats old or new, transferal of the old feats to new attributes under different names)...Vampire saw the same thing, with new sub-clan bloodlines capable of taking other clans disciplines (for example, blood magic began as the domain of a single clan, and by the end of 3rd edition, it seemed that virtually every clan had somee form of mystical ritual that could be unlocked through immortal vitae), level 5 disciplines all absically end up having the same effect even if they do manifest differently from a storytelling perspective (they work as a 'get out of jail free' card in a certain range of situations), merits and flaws designed to make character's interesting by circumventing rules with regard to specific attributes suddenly found analogues for every other attribute, and the interesting twists pretty much became the norm...Apocalypse World's specific moves that ask a narrative to be twisted in a certain direction suddenly become overwhelmed by numerous possible options regardless of where the narrative, no longer do the GM or players have to think before they can activate a move because the writer's of these spin-off hacks and expansions have just decided to throw in new moves that can be used anywhere (or have even suggested that GMs and players can just write their own moves on the fly...
...the exception becomes the norm. The chocolate chips become evenly spread through the ice-cream, and now it's just chocolate ice-cream with every mouthful homogeneous and the same as the last. I'm not going to say that this is better or worse, some people like the endless possibilities that these games reach at this level of "game maturity" (My personal opinion says worse but that's me). If your exceptions are going to become the norm, why not just include them as the norm from the beginning? It will save a lot of writing in the long run if you just write a basic procedure that covers a wide variety of possible exceptions. Or simply allow no exceptions.
In "System 4" I was thinking about the idea of warrior mages and assassins...and various other character types that combine skill sets from two different ability categories (where "warrior mages" combine combat and magic, while "assassins" combine combat and trickery/thievery). if I'm forcing characters to select a specific action stance at the start of a round (determined by their mind-set, and defining which dice to roll), maybe it would make sense to create a special exception where these dual-focused characters can gain the best of both worlds when it comes to their specialties. A warrior mage can stand in "combat stance", and gains no penalty for casting spells, or they can stand in "magic stance" and have no problems fighting...similarly, the assassin can use their successes equally effectively when engaging in fighting or trickery/thievery, regardless of which stance they are currently in.
As soon as I thought of it for these two character types, I instantly thought of it for theurgists (who might combine magic and prayer), tacticians (who might combine knowledge and combat), even politicians (who might combine diplomacy and thievery/trickery)...and where does it stop. It make more sense in my mind to create a general ability that anyone can purchase, then allow them to combine two ability categories of their choice. Instead of writing up 15 different exceptions, I'm writing a single subrule that beecomes available to everyone.
Instead of writing a specific high level ability for each class of character which has the same basic function to instantly escape a combat that's going bad (offering a magical version that teleports the caster a way, a diplomatic version that forces opponents away through foul words and cursing, a trickery/thievery option of blending into the shadows, etc...), why not just offer a general ability that be omes available once a character reaches a certain point at any one of the ability categories? Let the player define how this particular ability manifests in play for their character, but offer a few suggestions for how it might work specifically in regard to a couple of the categories (because I've found most players have trouble with a completely blank slate).
I know that doesn't sustain the "supplement treadmill" style of publishing, but it allows players to customise their characters quickly and uniformly because there's a common rule in place. A player chooses the exceptions that fit their character, rather than forcing a specific small range of exceptions that might not fit the core character concept.
I'm not advocating that everything be reduced to a single all-purpose set of rules. If there aren't going to be differences between the way combat work and magic works, why bother having two seperate categories at all. You could go ridiculously down that path and have a game where everything comes down to a single number defining the character's overall importance to the story. I still want combat to be different to thievery, magic to be different to prayer, knowledge to be different to diplomacy, everything to have it's own specific way of manipulating story that cannot be replicated by the others. These fundamental differences will be in the core rules, plain and simple, unfettered by the exceptions to come later.