Forgive me if this idea has been posted before...
How many times have you been in the middle of a battle (mission or PvP) and said, "I really wish I had *** skil" (where *** is any skill of your choice). Well, here is an idea for a skill that could do just that (I have tried to keep it as realistic as possible).
Signet of Knowledge. Cast Time 5 seconds. Signet of Knowledge is *permanently* replaced with known non-elite skill of primary profession. (*permanently* could mean the end of a mission, enter new explorable area, only in outpost, end of PvP match or some VERY long time). It could also be an elite skill and then could substitue for any skill, but that would be a little restrictive since you can only have one elite skill on your bar at a time.
This would allow a little bit of flexibility during battles and would be a circumstantial skill. Don't know how it would affect over-all balance, but it does cost you a slot and five seconds even if you used it for a standard skill.