When it comes to the themes and interpretations of the magician class in a
3e conversion I would suggest that certain ones (a “witch” or one of the
ones described as kits in 2e in the BoM) might be best expressed as
prestige classes or through a judicious use of metamagic feats rather than
as part of the magician class itself. Curses, for example, are part of one
might consider the stereotype of black magic. In 3e Bestow Curse is in the
transmutation school, which would normally not be allowed to a BR
magician. However, one could give access to that school (or just that
spell and ones like it) through a prestige class or a metamagic
feat. Picture, for example a prestige class available to magicians that
gave them access to a special ability called, say, “Dark Magic” that gave
them the effect of school specialization (one additional spell memorized
per day of the following list & +2 to spellcraft to learn spells of that
school) and access to the following spells normally not available to
magicians, including Bestow Curse and related spells. Because of the
progressive nature of that special ability (spells by level) the class need
not have a lot of other special abilities.
Generally, I think that`s the best way to go, but one could include various
BR-specific metamagic feats to allow magicians access to things. The
creation of magical traps, poisons, etc. might all be things available to
particular magicians.
Gary