WW,
PH, and
ST have all been in a genericly 'age of discovery' setting and it worked quite well. You have steam and gunpowder, navies and suitcoats, plus a crazy little kid and his razor-sharp sword.
If it were modern-ish era you'd probably see a neat dichotomy between a modern world suddenly defenseless and perplexed by monsters and ancient ruins more akin to earlier Zelda titles. Any highly-stylized setting would work if you think about it. Steampunk is the most likely choice, but then an impressionist or art-neuveux could work too. The stark reality of fantasy would be very well set against an impressionist modern era, especially if a big emphasis of the monster hoard was light and color. So, simulataneous with the enemy invasion is a metaphorical one imposing one art style on another. Extend that beyond just occupied lands into the enslaved people and you'd have a visible representaion of what the hoard is doing emotionally to the subjugated populace.
Actually, it would sort of be funny if investigating ruined temples had a kind of Egyptian-graverobber sort of theme, its art style much like those early film recordings.
Imagine the uses of a timeshift stone in that kind of game. Shifting between a modern city and an older fantasy setting could be pretty neat. Beyond just how awesome it would be to shift an impressionist city back (ironically) to a neo-Raphaelite kind of medieval setting, but it could serve the dual purpose of gaining you access beyond guards or getting into places that no longer exist or exist only in the modern era.