
Drawing psykers or sorcerers could be a good start
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I would look at existing example of
GW art for inspiration and have a look at some minis, for poses and to get a 'feel' for what they are about. Make no mistake, what ever your style your portrayal will have to fit with either the
40k or fantasy setting.
You could illustrate a psyker unleashing a blast of immolating fire at some heretics or make it a decrepit renegade zapping Imperial soldiers. Maybe have a Psyker succumbing to the influence of the warp on his/.her knees with warp fire spiralling out its orifices with a commissar ready to deliver the emperors mercy.
Have a chained psyker in front of the inquisition.
For fantasy you could have a wizard performing magic tricks or offering advice to an army general. Maybe summoning an epic lightning storm on a battle field?
Obviously these are Imperial/Empire ideas so they may not float your boat race wise or aesthetically either.