Um...I see a few things that is causing your problems.
I've been working on an expanded skill system. I'm adding a few skills, because I want to have Intelligence grant additional trained skills, and I'm granting proficiency bonus to all saves (and changing how classes give out save bonuses)
So first off, you’re adding complexity. That always causes problems. Second, a higher intelligence wouldn’t necessarily grant more skills. It could grant more proficiencies, but not necessarily skills. And there are character choices which grant more skills. If you wanna be a skill monkey, there are things to choose to do that. You can’t give everyone everything. Though it makes sense to have more tool/language proficiencies with a higher intelligence, as those are usually more intelligence related anyway.
but while I'm at it, I thought I could codify more of what skills can do. I don't like that cool skill actions are hidden away in the feats; many of these could be skill checks. Acrobatics check to stand up from prone quickly, athletics checks to climb faster or jump further, stealth checks to disappear when someone glances away. The skill system can be expanded to allow non-casters to do incredible things, the likes of mythological heroes.
Feats are the incredible things the likes of Mythological heroes could do. Skills are just skills that anyone could do, and skill checks are used if there is a chance it won’t work. There nothing stopping you from allowing a check in these cases, the feats merely allow it to happen with no additional checks. The Athlete Feat allows them to stand up with 5 ft of movement, no check needed.
Also, if you simply grant these abilities to anyone who asks, you’re basically handing out free Feats, which spits in the face of anyone who forwent another choice because they wanted a specific Feat for that ability.
But, bounded accuracy makes it so that skills with Expertise cannot be put up aside saving throws. Saves are Proficency+Stat, expertise is (2xProficiency)+Stat. A +17 vs. +11 is simply not fair, and that's assuming a common threat would have a +5 ability modifier on something like Wisdom if we were, say, adding a Demoralize "Intimidationg check vs. Wisdom save" mechanic.
I think this is your main misunderstanding. It’s not Intimidation vs Wisdom Save. It’s Intimidation vs Insight Check. Saves are against spells and attacks and the like. Skills are pitted against skills. If the target has expertise in Insight, then they can easily match someone with expertise in intimidation.
Same with Stealth. The contesting check is Perception, not a Wis save. So there’s that problem solved.
And that got me thinking that Expertise might have not been the best way to model how experts are better at certain skills. I mean it makes perfect sense, an expert is just better, right? But if an expert in thief's tools is the only way for the party to get past a lock, then only the expert in thief's tools in the party is going to get past it.
The DM decides the lock’s DC. The whole point of expertise is to do things no one else will do. It makes the DC 15 lock, which someone without expertise may find mildly difficult, into a cakewalk for the one with expertise, as it should be. A 20 DC lock would be hard for others, but only mildly so to the one with expertise. And yeah, the DC 25 lock is extremely hard for most, but not that hard for expertise. They’ll also be the only ones who can unlock a DC 30 lock, and at lvl 5, but in that case, the DM shouldn’t make a lock that high DC unless he really, really doesn’t want anyone to open it without a key.
Why can't expertise allow someone to do things faster (bonus action instead of an action)?
Making it a bonus action instead would do nothing for helping to unlock a lock, considering most people unlocking the lock will not be doing it in combat, so action economy means nothing, and arguably the Rogue’s Cunning Action could already make it a bonus action. So by doing that, you’re just nerfing the Rogue. There are other abilities that make standard actions into a bonus action, Cunning Action being the main one. If the character wants to do that, they should choose those abilities.
Why can't expertise allow someone to perform routine tasks easily under pressure (take 10, or reliable talent)?
They do. That’s literally what they do. At lvl 5, a Rogue can have a +11 on any skill they have expertise, which means they literally cannot roll less than a 12, unless they roll a 1, which for me is always a fail, though that’s a house rule. It also means that harder DC’s, such as 15, are trivial, or even 20, they are still better than 50%. At lvl 5. This is what Expertise is for.
Reliable Talent simply extends some of this to other skills that they’re proficient in but don’t have expertise. Basically, at lower levels, the Rogue can do this with a few skills. And at higher lvls, they can do it with more.
Heck, expertise could allow for double proficiency bonus BUT not ability score modifier; this would allow a rogue who is an expert in athletics to not need high Strength to perform incredible feats of athleticism while not needing a high strength (that would be out of character) ... I'm really liking that idea.
This doesn’t solve anything. Why would you erase the ability modifier? That’s literally treating the ability score as if it were a 10 or 11. Literally, a Rogue with an 11 Str would have the exact same Athletics Check as a Barbarian with the Prodigy Feat granting Expertise in Athletics, and in actuality, someone WITHOUT expertise, but a 20 in the relevant stat, would have a higher check than the one with Expertise until lvl 17. That makes 0 sense to me. You can already have a Rogue with an 8 str take expertise in Athletics and become almost as good as people who are stronger, why are you going to penalize those who are actually stronger?
When two people of equal skill come together in a test of strength, the stronger one will win, as the equal technical skill cancels each other out.
Instead of just "I can hit higher numbers", I think expertise should be "I can do cooler things". Bake in a mechanic like "power attack"; take -5 to your check, but you achieve something more with a success. A regular person wouldn't do this, but an expert could. Expertise could allow for doing those sort of actions without the -5 penalty, or ignoring up to a -5 penalty.
You’d need to explain this. They have this kind of mechanical for actual attacks, which takes a Feat, but how does this work with, say, lockpicking? Take a -5 for...what? Unlocking the lock? Quietly, which isn’t even needed? What about stealth? You hide...EVEN BETTER! It just doesn’t make sense.
Thing is, Expertise is for making checks easier to pass. Feats are meant to allow you to do cool things, usually without checks. No one character can get Expertise in everything, so there will be plenty of things that challenge them. Rogues get the most, with 4 (5 if they’re a human with the prodigy feat), but that just enhances their skill monkey usage. Expertise allows a character to shine in their very, very narrow field. It makes them important to the party. Trading expertise for what is essentially a Feat for every skill they have expertise in would either break the game, or be completely useless as the benefit is never used.
