Perhaps a better way to go about this would be to go skill by skill and try and find balance that way?
Well, as long as you're asking.. I've already gone skill-by-skill and come up with personal rules/mods to balance them out for my own gameplay. Here's what I came up with..
Note that I play OOO-Full, and have since forever. All my comments on skills are from the perspective of how they work in OOO-Full.
Block: Fine as is
Armorer: Underpowered because failure only costs you a repair hammer, and these are trivial to come by. Solution is to use only one hammer per day (and you can't rest more than 8 hours a day!), so people who have better armorer will be able to repair more per day.
Heavy Armor: Fine as is
Blunt: Fine as is
Blade: Fine as is
Athletics: Fine as is.
Hand to Hand: A bit underpowered even with the OOO buffs, but not so much it needs to be altered IMO
Destruction: Fine as is except you can run backwards pelting the enemy with spells with no risk. Fixed by No Running While Using Bows Or Spells
Alteration: Fine as is
Illusion: Overpowered in the Charm department, which can just about wholly replace same-level Speechcraft. My solution is to advance each Charm spell one mastery level: That is, Novice-level Charm spells now require Apprentice Illusion to cast, Apprentice-level Charms require Journeyman, etc.
Conjuration: A little funky because the enemy seems to be biased towards aggroing your summon instead of you. I think if the enemy kills 3 of your conjurations it should no longer aggro on them (since by then it would figure out that you're just going to resummon!). My personal rule is "one summon per combat" but a mod could make a better soluation.
Mysticism: Soul gems trivialize magic recharging, and even someone with 5 skill in Mysticism can trap any size soul. I made the following changes:
1. Your Mysticism skill determines what max size soul you can trap. Novice can trap petty souls, Apprentice lesser, Journeyman common, etc.
2. You can only use one soul gem per day. Otherwise there's still no point to raising your Mysticism to trap larger souls when you can just use 100 crab souls to recharge anything.
3. You cannot purchase filled soul gems. It's silly that you can buy a 800 soul gem for 100ish gold in the same mage's guild that will charge 800 gold to recharge it manually!
Restoration: Fine as is except you can run around healing yourself at no risk. Fixed with No Running While Using Bows Or Spells
Alchemy: Overpowered, the potions you can make are excellent and ingredients are VERY abundant! The fix: you can make only 8 potions per day. How many potions per battle you can use is restricted as well, see Acrobatics.
Security: It's too easy to open locks! I make it so anyone can open Very Easy/Easy, you need Apprentice for Average, Journeyman for Hard, Expert for Very Hard. The skeleton key increases security skill by 25 points rather than just opening everything. This fails to actually integrate the Security minigame, though. IMO a better way to solve the issue with Security being useless is to have the speed of the tumblers change depending on lock difficulty and security skill. Someone with 50 Security skill trying to open an Average lock should experience tumblers the way they are now. Every 25 extra points in Security and each lock level lower than Average should halve the tumbler speed. Every 25 fewer points in Security and each lock level higher than Average should double the speed. So someone with level 25 Security trying to open a Hard lock would encounter 4x speed pins! Good luck..
Sneak: Fine as is. I use it to determine how much I can burgle per day (Sneak * 0.1 weight worth of items) however Sneak is plenty useful without this.
Acrobatics: Useless. I give it two buffs:
1. You can only use one of each potion type (i.e. one health potion AND one mana potion AND one stamina potion etc) per battle. It's hard to arrange potions on your belt in places that will be handy during combat after all! But once per day for every 25 points you have in Acrobatics, you can perform an "acrobatic feat" and reset this counter, using another of each type of potion in the same combat. Very useful for emergencies.
2. I use Combat Fumbling (which can immobilize you for spending too much time in the game menus while in combat) with the following settings.. no minimum penalty, no leeway before penalty starts to accumulate, 5 second maximum penalty. The "Item menu" rate depends on my Acrobatics and the "Info Menu" rate depends on my Speechcraft. Novice Acrobatics gives me 0.5 item menu rate, Apprentice gives me 0.4, etc until Master which lowers it to 0.1. What this means in plain english is that I get much smaller immobilization penalties for browsing my inventory during combat, so a character skilled in Acrobatics can perform quick feats of weapon-changing, equipment-changing, scroll/potion readying, etc. Agility will decrease the item menu penalty even moreso (automatically done by the mod).
Light Armor: Fine as is.
Marksman: Same problem as Destruction, same solution.
Mercantile: Not strong enough, and even if it was, the gold supply is too ridiculous for Mercantile to matter. My personal rule: in order to sell an item of value x you must have (x/10) Mercantile skill. That is if I want to sell a sapphire worth 77 gold I need a Mercantile skill of 8 (to assess its price and sell it wisely). Potions of Cure Disease worth 140 gold would require Mercantile 14 to sell. This rule kills two birds with one stone: It sharply diminishes an overabundant gold supply and makes Mercantile useful.
Speechcraft: What a mess. No fewer than five changes were needed to make this viable:
1. Bribery is completely disabled.
2. You can only do one round of Speechcraft per person per day. If you can repeatedly Speechcraft someone then you can max out somebody's disposition easily just by playing the game over and over, even at Speechcraft level 5. More limited rounds makes it meaningfully easier to persuade someone if you have higher Speechcraft.
3. Illusion's charm spells are harder to cast, see Illusion
4. Fast Travel is disabled. However depending on your Speechcraft level you can "hitch a ride" from one city's stables to another. Going from one city to another in a direction towards the Imperial City takes 1 "travel point". Going from one city to another parallel to (orange road) or away from the imperial city costs two "travel points". Travel must be along established roads. Novice Speechcraft gives you 0 travel points per day (no fast travel) Apprentice gives you one (towards Imperial City only), Journeyman gives you 2, Expert 4, Master 8. Examples: Getting from Cheydinhal to Bruma costs 3 points, one to get to the IC and two to get from the IC to Bruma, so you need at least Expert. Getting from Anvil to Skingrad takes 2 points, since it's two cities' worth of travel towards the IC (Kvatch counts as a city, but you can't stop there, since there is no stables), so you need at least Journeyman.
5. The "Info Menu" setting for Combat Fumbling depends on your Speechcraft. It ranges from 0.5 to 0.1 depending on your rank (0.5 at Novice, and 0.1 less for every rank thereafter). Intelligence reduces the penalty further. In plain english, having high Speechcraft allows you to go to your spell menu and swap between spells with fewer combat delay penalties. Someone skilled in wordsmithing would be better at remembering the invocation for a rarely-used spell under pressure!