So overall, the responses have illustrated what I suspected was true... there was no hidden issue with just giving the longsword the Finesse property.
Now, I can understand all the other possible "solutions" are viable as well, but in the end I just wanted the simplest thing-- make longswords more prevalent again (without needing to rename swords, or create new ones, or flood the realm with magical ones etc.) So I think I will just add the Finesse property to the longsword and remove the rapier from the game. And truth be told... while I understand completely the worries about DEX-the-uber-stat, it's never been a real issue at my tables only because many of my players just enjoy the STR-based character concept and don't power to the point where they've realized that STR-based character aren't "worth it" compared to DEX-based ones. So in that regard, even if I allowed the longsword to be Finessed *and* Versatiled at the same time (making a 1d10 Finesse weapon option now available), I don't think it would impact my games at all. Almost all of my DEX-based melee users either dual-wield or sword 'n board. So I'd actually be curious if any of them decided to try the main hand / no off-hand build for a DEX character if they could get the 1d10 damage. I'm not so sure any of them actually would.
Thanks to everyone for your thoughts! Much appreciated!
Now, I can understand all the other possible "solutions" are viable as well, but in the end I just wanted the simplest thing-- make longswords more prevalent again (without needing to rename swords, or create new ones, or flood the realm with magical ones etc.) So I think I will just add the Finesse property to the longsword and remove the rapier from the game. And truth be told... while I understand completely the worries about DEX-the-uber-stat, it's never been a real issue at my tables only because many of my players just enjoy the STR-based character concept and don't power to the point where they've realized that STR-based character aren't "worth it" compared to DEX-based ones. So in that regard, even if I allowed the longsword to be Finessed *and* Versatiled at the same time (making a 1d10 Finesse weapon option now available), I don't think it would impact my games at all. Almost all of my DEX-based melee users either dual-wield or sword 'n board. So I'd actually be curious if any of them decided to try the main hand / no off-hand build for a DEX character if they could get the 1d10 damage. I'm not so sure any of them actually would.
Thanks to everyone for your thoughts! Much appreciated!