I'd like something like you can make a pure class and be uber in that class or make a hybrid like ou can become a pure mage and be uber powerful in magic but screwed if it comes to something else or you could do the same for fighting or uhh roguery
stealth stuff, or you could make a hybrid where you can't get the best of the best of the best in both classes but you'll be above average in them, or a three way hybrid where you'll be average in all three or above average in one and below-average in the other two or something like that. But you can't be super uber in all three classes at once.
Oh and maybe have something were if you decide to become a masterswords man and go kill 5000 goblins like the dude up there said your other skills get rusty and you're not as good with them so if you want to be awesome at something you have to constantly use it in whatever you're doing but if you want to practice with all three you'll only be ok with them like with the hybrid. The more you practice with one the worse you are with another but it'll never go down completely fi you get high enough because it'll be like an muscle memory or whatever you call it. Like someone says i havnt done this in a while let me see if i remember how and it may take a while but they can do it.
Lets take lockpicking for example. you practice it for months and months and you can break any lock in 3 seconds flat or whatevers fast for a lockpick but you take time off to become a mage and you practice at that maybe occasionally using your lockpicking and you become an ok mage then someone from your theiving days asks you to join their crew and you go to show off to the boss that you can pick any lock in 3 seconds but it takes 15 because you havnt been using it for months, but you practice a while and it come back to you.
Oh and maybe for the thing above if youre skill goes down it levels faster than a skill you've never used because you've done it before so until you hit the level you had previously its faster but gets slower and slower til you hit the level you were and its back to normal leveling speed.