Yes, I know Minsc and his little friend.
Pyro + Geo is a common combination because the oil from Geo can be ignited by Pyro. Witchcraft (e.g. Oath of Malediction) is very useful for crowd control and (de-)buffing.
One of the best things about DOS is that you don't have to decide your character's whole career at the beginning, as there are no fixed classes with exclusive abilities. The "classes" of DOS are no more than themed presets.
For example, in our co-op game we have two magic users – an Aero/Pyro/Witch and an Aero/Geo/Hydro controlled by me, my partner has a pure Scoundrel and a pure Expert Marksman, so we have all skill sets but Man-at-Arms at our disposal. BUT even now with level 14 we decided to give at least two of us Man-at-Arms 1 mainly for Helping Hand, since no other skill group has a way to get up knocked down characters (well, Teleport does that but inevitably damages the target and has a comparatively high AP cost).
The best advice I can give you to lean back and don't overthink your builds. At least in normal difficulty, DOS is very forgiving and doesn't punish less-optimal builds (much
). Have fun experimenting and finding your own way of playing.