As for mages, actually for all chars, to receive the best benefits you really can only choose one of the traits it seems. You just will not level up high enough to get to 50 points on more than one trait. So, if you split it up amongst air, earth, fire and ice, you will not be able to use any of the higher levels spells. Now, which trait you choose is based on preference.
For rogues, unless your rogue is going to be on the front line, assassinate is useless. Better to focus on missile.
Just curious, what's your character levels by/close to the end game? It would help me plan out my build if I can estimate how much total skill points I will have.
Seems difficult to focus on just one trait, since front line needs both attack and defense skill, and mage probably need the main element + spellcraft.
Had to restart a couple of time because by level 3~4, my front line seems either underpowered in offense, or lacks defense....tough to find a good balance.