Perhaps I need to work on how my posts can come-off as being long-winded. In either case, here's the second part of how I feel the SPECIAL stats should have worked. Now, as people might notice from the first post, I have a few theories when it comes to designing a game: The first is that everything should have a purpose (this is indicated by my wanting Strength to be needed for more than just Melee-based characters).
I could go into more detail on it, but it'd probably turn into a tangent when you're here to read about Fallout 3 theorycrafting (though I'll make some posts explaining my different rules and theories for what makes a good game in the future), so I'll just jump right into it with the last 4 SPECIAL stats.
Hoo-boy... Charisma is a painful stat in that a lot of changes would likely have to be made to the workings of the game in order to make it feel like a really viable choice. The only difference that Charisma makes as a stat is that it modifies your Speech challenges (I've tested this personally, using the console command to test the success chance with the same amount of Speech skill with 1 and with 10 Charisma). This is well and good, but it feels very bland when people still generally treat you the same.
The most basic thing I would have done that wouldn't have been too painful, would be to modify the random things that un-named NPCs say when you try to talk to them. Seems that regardless of the fact that I'm running around with 10 Charisma, high Karma, and that I disarmed the bomb, people in Megaton still regard me as a total stranger. It's a little thing, but it makes the world feel less dynamic when I've done so much for this little town, yet the random locals still blow me off when I talk to them. Yeah sure there's that one chick who gives me stuff, but she does that regardless of Charisma.
Beyond that there are other changes that could be made, mostly in the range of different things that characters say to you. Maybe even the possiblitity of some quests only being accessible if you have a set amount of Charisma. If the character who gives the quest doesn't see you as "likable" enough, they won't give you the quest. However, I wouldn't do that for more than one or two quests to avoid it feeling like a cheap gimmick to make Charisma "useful".
Admittedly though, Charisma basically requires that the entire game be made so that you can talk your way through it to be viable, and even in that situation, only a Speach-based character would really care. Even in Fallout 1 they had to specifically design the last boss of the game so that you have the option to talk him down rather than gun him down just to make speach a more viable thing to aim for. So I don't fault Charisma's short-comings as much as I do the other stats.
This stat probably got stripped-down the most since the earlier days of the Fallout series. Like with Strength, I have to go back to the roots for the first thing I feel Intellect should have effected: Speech choices. In Fallouts 1 and 2, you had more or less speech options based on how smart your character was, and his charisma effected how the characters reacted to those options. The two thus worked hand-in-hand if you wanted to build a "Speech Character", as you'd need high Int to get the options, and high Charisma to increase the chances the NPCs would care about what you're saying.
Expanding on that, I also feel that Int should play a roll in some of the Skills it's claimed to modify. As an example, when repairing gear, a higher Int should play a roll in how much you can repair. Let's just say that you maxed your Repair to 100. If you had low Int, you'd still only be able to repair upto, say, 90% of the weapons durability (as it is now, 100 Repair lets you repair upto 100% durability). It's a random number I pulled at the top of my head, but a general idea of how Int could become a more viable stat when wanting to repair gear to a higher level.
For Science, it would work something like this for hacking (I'm going to assume you know how the hacking works so I don't have to go into detail). At 6 Int, when you pick a word from the list, the game would highlight all other options that could match based on the number of similar letters. At 8 Int, it would also look at the second word and further eliminate options. Then, only at 10 Int would the game further narrow it down when you've selected a third letter. This would make Int a valuable stat for anyone who wants to focus on hacking computers by making it less of a squint-athon to compare letters.
With Medicine, it could factor into healing your limbs. First, I'd reduce the default amount you're able to heal the limbs to about 60% of the current amount. From there, Int would give a 5% bonus (multiplicitively) per point, upto a max of 50% when maxed, allowing for roughly 90% of the current limb healing amount (which is still really high mind you). This would make healing limbs a little more punishing for anyone who decided to slack on their Int.
Oh, and I would get rid of the Skill Points modifier, and just set everyone to gain 15 Skill Points per level. As the game is right now, Int is a little too powerful early-game when it can be used to get a lot of Skill Points, and completely useless the instant you reach max level since it doesn't modify anything beyond giving more Skill Points when you don't level up.
In the current game, Agility only modifies the number of Action Points you can use during VATS. This is phenominal for Xbox players, and ho-hum for PC players. One idea is to create a "Dodge" stat. Every point of Agility gives gives +2% chance for a melee-based attack to completely miss you, maybe even make it so using a ranged weapon cuts your dodge chance by some percentage. Perhaps a little less if gear or perks were to be added that can also increase dodging. Another thought is to have Agility modify your run speed slightly. At 5 Agility (considered average), you would be at the current normal running speed, and every point of Agility could add or subtract 2% running speed, putting you at 110% running speed when maxed, and 92% when lowered to minimum.
Honestly, I would leave Luck as it is. In the current game, people already usually try and get their Luck maxed for the Crit modifier, mostly due to a lack of anything interesting from maxing the other stats in the current game. So were the aforementioned changes put through, then Luck would be about on par. People would still want points in it, but at the same time they'd actually be sacrificing something to get that extra crit.
Perhaps one thing I would alter is that I would have every point of Luck give a 1% added effectiveness to everything else that every other stat can effect. This would make it so that a max of 10 Luck would be like having one extra point in each other SPECIAL stat, even if that stat is already maxed, kind of like the stat having a hidden 11/10 value.
That will do for now. Next week I'm going to wrap this up with something of a run-down on different types of common character builds, and how the changes I proposed would make for more interesting choices when choosing your SPECIAL stats at the beginning of the game.