This is something 1st and 2nd edition got right -- if you want your fighters to do things that mages normally do without destroying the class you give the fighter armor and weapons that have spell like effects.
What about helmet full of diamonds can be used to casts fireballs? Leather armor that allows you to cast misty step? A sword that duplicates the effect of vampric touch? And so on.
But, in any case, the weapons master is the strongest class in the game right now. Awfully hard to beat that Gith patrol without Lae'zel in the party . . .
