If i were you, i'd keep Minsc (hes still the best non-evil fighter type) and consider a cleric or druid.Jaihera is an excellent choice, as she is a good druid and a good fighter, Anomen is about on the same level, but Cernd is absolutely AWESOME for magic support.
If you want a better mage, i'm afraid that there isnt one (sorry) Imoen is an average mage, Nalia is crap (sorry but its true), Aerie is ok, but a bit on the weak-as-a-lamb side and Edwin is an evil b*****d. What about dualling your Human fighter to a mage? (he will need a high INT score). Level 12 is a good time in my opinion.
Are you using the TeamBG mod The Darkest Day? (URL given below) If you are then there is a cool wizard named Robillard.
The Darkest Day webpage