1- By 1870 the Austrian Empire (explicitly mentioned in the novel) didnt exist anymore.
2. Carmilla didnt get locked in the coffin. Sleeping there was just part of her vampiric curse but she moved around freely
3. In Castlevania, Carmilla already had Laura in 1792.
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The best moment to have a Carmilla story in CVerse is before Rondo. Carmilla's novel events happen around 1770 -1780 in this version while the game events happen just before RoB. Shaft's cult (Which Laura joined and became a high ranked priestess) ressurects her to help them track down Drac's coffin/remains. First part of the game would be against the Renard vampire hunters (Carmilla delivers Maria to Shaft) and the later half would be against Carmilla's mother and grandmother from the novel (the ones actually holding the coffin, they framed the Renards) who basically pull a Brauner and raise the Castle. The Grandmother would be the final boss.
Mechanic-wise it'd be similar to OoE. Two "weapon" slots that consume MP but instead of just a trigger we'd have two (since consoles have 4 triggers): One would be "Laura" skills and the other would be the typical magic stuff like turning into a wolf or a bat. Instead of item crashs i thought of a demon form too that would turn Carmilla into her typical bossfight form.
Like OoE too we'd have a base (Ecclesia) which would be Carmilla's waterfall fortress/Shaft cult (unlike Ecclesia, it'd actually have npcs and quest givers) base, a city central hub (Wygol) with quest givers and flavor npcs too and a variety of locations you can go to from there.
If the game is 2.5D it could have extra costumes like LoS Carmilla and Laura or Annette and Maria
Well then again I originally also had in mind of something existing outside of Castlevania canon and it's a stand alone retelling of some sorts but thanks for reminding me of the chronological errors though but I guess like I said before, it could be a revision set in the 1870s just like "Kinda TV's" Carmilla is set in modern times.
Well to tell you the truth, the real reason why I prefer the 1870s is because thing is in the 1790s however, the gown designs from the OP doesn't look exactly accurate to how dresses from the 1790s actually
looked like which hers looks like 1870s/1880s since to be honest, I find Victorian dresses/gowns more aesthetically appealing than gowns from the 1700s especially late 1700/1810s they look like nightgowns which isn't aesthetically appealing (and attractive of course) and less elegant than the dresses in the late 19th century are. Unless there could be work arounds though somehow....
Of course I've just rechecked Carmilla's story, it seems her ballgown appearance defiantly fits the original story because she regularly attends Masquerade balls (since during those times, they were the nightclubs/gothclubs, raveclubs of the day especially for high society...since it makes sense that modern vampires would search out prey in rave clubs just like the masquerade balls of the days of old) which is how she feeds on human prey of course so it makes sense in the context of the original story.
Now the actual reason I'm envisioning this game as nothing to do with Castlevania's canon is well I just happen to look at Carmilla's info and turns out she's a complete villain type character (well maybe I don't mind this at all because she would be seen as a villain to hunters anyways) and Laura which turns out that she doesn't want to become a vampire and 'feels embarrassed and intimidated' by Carmilla as said in the wikia a far cry what I envisioned since it would have made her less sympathetic if there was any hint of toxicity in their relationship which quickly defeats the entire purpose there which is what I'm trying to avoid.
Also Laura in a cat ninja suit (if she's attacked) was off putting.