
A side effect of this would be that the melee char will need a caster for his physical immune mobs and the caster char will need the melee for her spell immune mobs.

Oh yeah, to further this, maybe mobs that have physical immunity can have spell sensitivity and vica versa. In this way parties are encouraged even more.
Just be sure to have regions where there is one or the other (obviously all the mobs will be worth less xp in these regions) and regions where the two intermingle (the xp is higher since no caster/fighter can be versatile enough to deal with everything). let's penalize the versatility of multiclass by making the most powerful items include a script that says if you are anything but this character you cannot use the item (not useable by:)

