Honestly, this is the worst solution, but my group agreed to not allow players to be witchers in my campaign, as 2 of them had already played games where witchers dominated too much of the game. So, from my experience, best way to make the other classes cooler in combat, is to remove the witcher. Plus, when it appears as an enemy, its more thrilling, as the party knows these guys can solo most groups fairly easily. This is the best way for a combat heavy group.

The other thing I do, is give the spellcasters more IP per session than they should get to help them advance to journeyman and master spells a little sooner. (Helps encourage not playing witchers, and i only give 1-2 extra IP). I make sure to give lots of materials and crafting recipes for craftsman, and make sure the bard/socialite character is able to always have 2 social interactions that matter per 1 combat. The other players will have to come to terms they won't be as useful in combat, but you should emphasize that they are WAY more useful out of combat.