Is Academics useful? Language? Seamanship? Does one really have to use Common Knowledge?
The rule is always "if you don't care about a thing, it's pointless to bother tracking it." But I would offer the following replies:
Academics - This skill covers everything from Research (the archives kind), to Law, to Writing, to Economics and Business. Basically, if it's a college degree that isn't art or science, this is where it lands. A Professor of Archaeology, a Lawyer, and a Journalist would all have training in Academics.
Language - Unlike
most skills, I would argue that the fun way to handle language is binary, so this is actually a catch-all term for a pile of languages, and you either know a language or you don't. Don't care about it? Ditch it.
Seamanship - This is a situationally useful skill, depending on the campaign. It covers handling sailboats, canoes, kayaks, navigating, and so on. It could be hand-waved in most campaigns, especially if PCs leave the heavy-lifting to hired hands, but if you tend to run pirate campaigns, it probably shouldn't be.
Are some of these replaceable? Can I do the same things with one or more of these skills? Can I conceal a stolen purse with stealth or thievery or bluff? Can I rally troops with language, performance, or persuasion? If I have no fighting skill, can I use athletics instead?
I can't tell if you're trying to be difficult or genuinely confused.
"Stealth" is hiding. "Thievery" is the catch-all for picking locks, disarming traps, and picking pockets because they're the skills required for being a "Rogue," "Thief," "Burgle-Hobbit," or whatever you want to call it.
"Bluff" is one of my "Social skills" (along with Intimidation and Persuasion) which I could make more obvious by grouping them, but I was listing things alphabetically. I like the idea of drawing a line between am I trying to scare someone, convince them, or get away with a lie. I think it's interesting to have PCs decide what their approach is, so I want three skills. A Paladin who threatens to torture someone (but doesn't plan to follow through on it) is engaging in "Bluff," not "Intimidate." That's a very different thing to be good at.
"Performance" = Performing Arts. That would include acting, dancing, singing, and playing musical instruments with the intent to provoke an emotional response. And probably a few other things. A good dancer is probably trained in Athletics too. I'd say a good actor probably also needs Bluff to be convincing, but maybe that's an argument that Acting shouldn't be here. But from the "what's the intention?" thing, maybe it should be. But as for dancing, think of it like how they score Gymnastics or Figure-Skating at the Olympics - Technical Merit: Athletics. Artistic Merit? Performance.
"Athletics" is simply the catch-all term for things not covered by other physical skills, like climbing, swimming, running, jumping and acrobatics. Fighting? Sorry, separate skill. No system is perfect. But what we don't need is a "Skiing" skill when "Athletics" will do fine.
This sort of tells me that the 5 Core Skills don't belong in the same skill system, because it's assumed everyone can do them. That breaks up the definition of "skills"; do they require training, set characters apart, represent something that you can't really do without some level of development with them?
I feel like this needs clarifying. If the skill system in question only has "trained" and "not," you would be correct. In a level-based game, the "Core Skills" aren't necessary. They're intended to reflect novice level competence that all adventurers acquire in the course of growing up.
However, I personally hate systems that try to put all the heavy lifting of skill differentiation on attributes. Sure, native talent matters, but so does training. This system would need modifying for something like D&D where all your skills simply improve whenever you "go up in level."
Common Knowledge exists to cover the basic things most character in a setting would know - local history, cooking, religions, etc. This is also where things like reading, writing, and basic arithmetic go. It's also where a lot of the marginally useful skills (cooking, for example) would land. Could many parts of this one in particular be covered by an untrained check in some more applicable skill? I guess, but "can do basic math perfectly fine but struggles with science" and similar examples are a common enough condition that it feels like it needs representation. And since we already need a "where the hell does cooking go?" category, let's let it do more. One could also have "common knowledge" reflect "background knowledge" the character has when the campaign starts, and vary from PC to PC. The farmer probably has a different "common knowledge" skillset than the guy who was a sailor.
As a final note: no matter how clear I think it is, a clever person is ALWAYS going to be able to make arguments for crossover applicability, especially if all you have are skill names, not descriptions. I've thought the list through pretty thoroughly, but it might not be compact enough for everyone. And I'm sure someone is going to be able to pull out a "But what about..."