I don't want to sound too sjw-ist, but:
Genocide and generically pushing the idea of one-sided progress.
No, seriously. The frontier myth is based on the idea of the natives being not-totally human, people without agency. And the "good savage" myth is this as much as the "bad savage" myth in this regard, interestingly enough.
Also: cities means corruption. The frontier means perhaps danger, but it's simpler, purer.
Nature isn't really "the enemy" (this is almost unheard of in fantasy nowdays, but still): or at least it's not that much feared, it's more a source of awe, perhaps a crucible for the chosen ones. Interestingly when the west was at it's golden age, people were already decrying the death of wilderness in the US.
Religion is an interesting idea here, because while the christian ideas were univeral there never was a unified church for all. And to an extent the churches in America were always bottom-up, more than in Europe anyway.
Progress might be an interesting to play with, but you might fall into the trappings of stereotypical magitech (basically our old tech with some special effects) easily.
Possibly the race issue (i always felt DND in particular was heavily touched by that, deep inside). What if there is a huge influx of overseas (extraplanar, whatever) halfling working at your local magitech rail? What if there is a war about orc slavery?
>I like the idea of a magical civil war because it could mean an idea of society vs another one
And overall, oddly enough: republican ideas. I mean, in fantasy you always see kings, dukes, bishops, perhaps university people or mechants in charge (and the upper-middle class is criminally underused in "european" fantasy politcs anyway but that's another point).
Consider this: your average town under your average evulz undead horde approaching. Better yet, just rumors at the time. Are you heroes bad enough dudes to actually get Joe the half orc crazy farmer work with Beth the elven preacher?