Of those games you've listed, I've only played Divinity and Diablo. Diablo 2 has one of the best skill systems, mostly because it's specialised for the game style: hack & slash. Otherwise, the Dungeons & Dragons skill system.

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Also the skills would have a cap dependent on either (or both) your level or your attributes. So at level 1 you couln't train your sword slashing past skill 20 for example until you either level up or your dexterity is at 16.

Yup. That's a good way to do it. Even better, I'd like to see an attribute-less system. Just have skills, some of which cross-teach each other. (eg. Improving Fire ball slightly improves every other fire based spell.)

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I also think there should be a manner by which you can forget skills you previously learned if you neglect them. So if your just hacking and slashing all the time, and you never train your pickpocketing then your pickpocketing skill should slowly diminish.

I don't agree. Skills points are usually a rare commodity. I'd hate to lose all that I'd learned because I was in a combat heavy area or got stuck at a puzzle for a long time. Or even because I forgot to pause and went to make dinner!

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Another idea would be that your skill points would be totally open (ie. you could train any skill to its highest level from the outset), but as the skills get better and better it takes longer and longer amounts of time to increase it, and attributes make it more easy (ie. less time) to train skills at higher levels. Leveling would only unlock percs and attributes.

Sounds good, but what about skills like Alchemy and Lockpick? What bonus do they get once you are able to make the biggest potion or pick the most complex of locks?