Frist of all bring back pass/fail if fail you get the worst possible outcome for what you are trying to do or get the item in question destroyed. Also in Fallout enchanting is replaced via Science based stuff. Also you should be able to learn all the skills/preks brought up in this, using Skyrim as a base if its 3 or 4 paths to upgrade the first path is easy the next path is hard the third and fourth path is hardest. I really hate locking out paths but some difficulty mechanic is not so bad an idea..

Basically I would like to see a system where you can stack status’s and effects. Say you want better lather armor outside of remaking it as a higher quality set you should be able to apply some basic metal and leather bits to enhance its protection stats. So the main stats for armor is defense, magic defensive via natural material(or for Fallout effect protection via conventional non super science skills), weight, speed bonus/penalty and condition/degradation if applicable. You could break these up into categories to refine each of their bonuses.

You always have a chance however small to apply positive stats but most of the time you are going to get penalties with few bonuses, the higher level you are and the better your material is you have a better chance of applying positive stats. The ratio should be something like 1 in 200-500 for all positive stats starting out. You can also take stuff apart with a 90% fail rate starting out, to fail means you get a few basic items from whatever you took apart to successfully pass it means you get the best possible materials of tis construction from it.

Enchanting works in a similar fashion, albeit you can transfer an enchantment to a blank scroll this process has a high fail rate starting out leaving the piece of equipment highly damaged, the better the scroll and your skill level the better the possibility of error free transfer.

Spells work like skills in that you transfer a spell you gain a small amount of skill like points to that spell general type IE all fire go to fire the more you do it the higher your knowledge of that spell making it easier to apply and transfer.

No spell or blessing is immune to this transfer process however its broken up into 3 categories unknown with a 99% fail rate at very basic starting point level, blessing which are at a 75% and normal spells which are around 45%.

Stacking enchantments is rather easy if you succeed in applying a transfer you get a very small amount of that spell applied to whatever you are enchanting. The higher the spell level the higher the amount you learn of that spell type. Enchanting is broken down into 4 categories Affect(time in use, radius,constant effect,ect)Aquire(used to select specific targets of the spell like general types of undead, giant, dragon, light armor-FO, heavy armor-FO, anti personal-FO, anti armor-FO, ect) Protection variations and Offensive variations.

So if you have fire or healing, constant effect and radius you can make a flaming item that dose damage to anything near you.

Now for the fun bit whatever it takes apply and raise the stats/effects on equipment its 3 times harder to apply a NEW effect or stat, a fail in this will result in you turning your carefully tweaked armor into poo. LOL

edit

And now I recall something, instead of a hard fail where the thing is destroyed you do damage to its bonuses and raise its penalties , so you keep working away at it losing only consumable items that are reasonably replaced, the more rare it is the better your chance to raise positive stats.. For adding new stats it gets it gets twice as hard per new stat. That's a better system, maybe make a hard fail rare and kinda random based on your combined skills and equipment,ect.

In my opinion, "pass/fail" is an overly simplistic and frustrating game mechanic. A system that does a skill check, and uses scaled results depending on the margin of success or failure creates far less temptation to reload until it works. Just as importantly, the difficulty of the task should affect the odds of success, so if I'm attempting to produce a Novice level potion or basic piece of leather armor, the odds should be far better than if I'm attempting to make a Journeyman level potion or produce a Fine Quality steel piece. Success or failure could range from producing one step better than expected to producing scrap, with most attempts creating something in between.

For example, you are at Apprentice level, and try to create an Apprentice level item. Your skill is on par with the difficulty, so you're most likely going to either marginally succeed or marginally fail. A marginal success would mean that you make what was intended, and a marginal failure would mean that it's not quite up to the grade that you attempted. A really bad attempt might lead to a complete failure or two grades lower, and a really successful check could mean that the item is a grade above what you hoped to make. Attempting to make things one grade above your level would lead to sharply increased rates of failure and moderate chances of total failure, and practically no hope of doing better than intended, while attempting a grade lower would be almost an automatic success.

By giving the option to produce different "grades", with chances of success or failure, rather than scaling the output to your skill level, the vast majority of the results would be stackable with "standard" items. You might occasionally get a piece with a unique flaw or advantage, but 95% of it would fall into the existing "grades" of items in the game.

This is true and my numbers will always be off anyway, but I was thinking leveling up plus perks plus better materials means whatever you are doing that 95% fail rate becomes more like 70 and for highest level game breaking enchantments that's not such a bad thing. Also 3 to 5 out of ten to pass fail most things starting out is not such a bad starting point but that's un augmented by tool/equipment,work area and materials. I was thinking some sort of simple 3-5 out of ten pass fail setup for first pass then do some sort some sort of 20X0-10(0-200%) roll minus some sort of penalty roll that reduces that number but gets better as you grow that brows into a 50% bonus at max level so the stats you do is 250 of whatever spell range or average for the equipment but there again I am rambleing about numbers that make no sense...

But you are right fail rates need to be rare for most things and now I recall something, instead of a hard fail where the thing is destroyed you do damage to its bonuses and raise its penalties , so you keep working away at it losing only consumable items that are reasonably replaced, the more rare it is the better your chance to raise positive stats.. For adding new stats it gets it gets twice as hard per new stat. Dose that make more sense?