If they made a one off expansion that was just completely outside the box of High Fantasy, I would simply ignore the expansion. If it added some new mechanic that changed the game (maybe a Must Have Spell or some entirely new trait/defense) then that would be the likely end of my continued interest in the game. Speaking PURELY hypothetically, of course.
Now if they decided to have some light steam punk mage, or some gnomish artificer with guns and automatons... I'd have to see how it was done. My initial "gut reaction" is to avoid that dilution of the high fantasy frame, but it could be an expansion rather than dilution.
Basically, I want to feel like I just stepped into the Arena with my trusty book of spells to duel. That's what the game's premise is advertised, and I believe it achieves it very well. Once you start matching up my Gorgon Archers or Lord of Fire versus Gnomish Longrifle Brigadier or Voltron... I cease to be interested.
I should say if that if the game were based on a clash of futuristic technomancers slinging code and gauss rifle rounds at each other, I would completely avoid/dislike any filthy fantasy elements too. I want my archetypes pure dammit.