I am not big on history, so please correct me if I am wrong, but the rapier is from a different place than the standard long and short swords, right? Have you considered weapons from other countries or areas that may have traded with south Asia, that could be d8 and finesse-able, but still fit thematically?
No, the Rapiers are European, same as "longswords" and such. They are from a rather different time period than the usual mediaeval range that D&D settings tend to depict however.
That said: rapiers and "D&D longswords" were used in the same time periods: Rapiers were used as fashion accessories and for duelling, but generally a real sword would be used for actual combat use where you would be fighting to kill or disable, against a foe who might be wearing armour, and in a disorganised melee.
I don't get the idea that rapiers would be civilian. During the 30-year was I think rapier was the standard sword in the Swedish army. And even if it is more civilian that also makes sense to me for adventurers.
Civilian in the sense that it was generally carried to be seen and to advertise your family's fencing master, and in honour duels with codified rules. Adventurers would generally use military weapons, which are designed to be used in a real fight.
Of course, there are times when clanking down the street like two tons of shiny metal death looking for someone to happen to is illegal, or even worse, a social
faux pas. Where rapiers are the fashion, it may be best to follow fashion.
Yeah, I replace rapier on the weapons list for my generically medieval campaigns with the medieval knightly sword/ later medieval arming sword, of which the rapier is just a further evolution anyway. I dislike rapier because it's modern and distinctly civilian in character.
Maybe, but I'm not sure I'd consider those swords finessable.
Just looking up the Swedish Imperial Army on Wikipedia, I see that musketeers and some pikemen used the rapier for close quarters defense, whereas cavalry were issued the broadsword. I find that generally, however, the rapier is considered a "dress sword", used primarily for self defense in a civilian context.
The rapier was likely used because it was very much a backup weapon after you were no longer able to use your musket/bayonet or pike and they likely owned one for civilian use.
The reason that the cavalry were issued broadswords instead is that it was their primary weapon: they were expected to fight in melee using it.