[quote=@Oak7ree]
Could there be magic-inhibiting substances, anomalies, creatures or materials?
[/quote]

Some ideas I had where:
- The realm from which the mages draw power from (lets call it "Arcanum" for right now) could be a fickle thing and waxes and wanes in terms of how strong it is.

- Some illegal drugs could possible have a strong sedative properity that interfers with magic. Hell, sedatives or stimulants in general should make it harder since they will mess with your brain.

- Using certain forbidden or extremely powerful spells has a chance of conjuring up a magic demon beast or something and not even master mages can completely negate that chance.

- There should be some normal beasts that can "see/feel/sense" magic and thus can track people, espeically mages, across otherwise impossible terrain and hunt them.

I have some more ideas on materials, but that's another list in it of itself.