I want to give my players a lot of dimensions and options to be clever.
I've already given the spellcaster a horrible device that I read about on reddit: A ring of the Grammarian (allows you to change one letter of a spell's name to change the effect). That was already interesting when he once cast "Firebaal" instead of Fireball.
I want to give each character a ring that's part of a multi-ring set. The general working idea is that it allows each player to "borrow" an ability from another. Not a race ability but a class ability. For example, the thief might be able to "steal" the wizard's "blink" spell. The ranger might be able to borrow the backstab ability from the thief. And the wizard might be able to borrow "pick lock" when he fizzles on a knock spell. And so on.
The idea isn't to give them power but to coordinate more during combat and tactical situations where the players must line up their actions to build to a crescendo action. Get them communicating and planning, really get into their character more.
Thoughts?
How does the owner of a skill get it back? Is it copied, time based, taken back, given back? All would work, but you should consider if you're making that clear in the beginning or communicate it when it's probably too late.