Hmm, good question.
The Var Attre twins would've been a nice opportunity - I do agree with the folks who have said in this thread that there needs to be some storyline context, not just sex for sex, though, that's always the tougher part.
So...
Var Attre Twins
Couple Inn Keepers I thought would've been nice to have a romance option with -
Inn Keeper at Fox Hollow in Toussaint I thought was very attractive and her conversation with Geralt she won't stop basically worshiping him for saving her town from the bandits in the area - would've been nice to be able to respond with something like 'well, hey, if you're not married, I can think of a great way for you to thank me', and that could lead to something more, even if unintended by Geralt originally.
Here's a pic of her also showing the Inn itself (and Triss is obviously there on the right as well):
Here's a closeup:
And here's the Innkeeper at the Inn in Beauclair where you play in the Gwent Tournament (can't remember the name of it, sry)
Anarietta would've been my top option with Dandelion out of the picture in Toussaint. After Geralt shows up and basically saves her and her country, I would think she'd move on from being besotted with a dorky dude like Dandelion and want to try getting it on with a warrior type like Geralt. She is easily the prettiest girl in the game, IMO.
Cerys en Craite - that's a big one for me, too, probably right below Anarietta. Yes, it's your friend's daughter, but you know what, she's an adult, and in my version ends up ruling Skellige. And she spends quite a bit of conversation time being somewhat flirtatious with Geralt while they are working together. Never has a woman with a big scar on her face looked so damn attractive to me.
Vivienne possibly but she was somewhat emotionally damaged given all the trauma she'd been through with the curse and all, so I could see why she just wanted to skip town and go do some traveling, good call on that one developers.
Not in the game, but I thought it would've been neat to have a side quest involving a very attractive elven girl (most of them were not very attractive in the game, IMO, with the exception of the elven girl at the brothel in northern Novigrad, but she doesn't count since it wouldn't really be an emotionally involved romance) at some point just to add exposure to the different races, etc.
Fringilla Vigo - this was just a missed opportunity altogether in the game - should've had more screentime with Geralt to potentially apologize for having attempted to trick him previously (from the books), explain herself, etc. They never really talked, Fringilla just basically teleports away from him when he tries to talk to her, so I thought that was a missed opportunity overall with her character.
Madame Sasha / Carthia Van Canten - I know you get to romance her to a degree, but thought this one was quite intriguing. Beautiful girl who was a spy for Nilfgaard - would've been a quite a bit to explore there if they'd chosen to develop her character and a potential romance, there - could've had some nice add-on quests with her as well, perhaps she needs assistance from Geralt to get to a ship and avoid notice, etc. A DLC package could've taken Geralt to Nilfgaard. (that's a place we've never been, would've been very exciting)
Louisa La Valette is a good one if you saved her son in W2. The backstory on that one is already pre-written and easy as pie to implement, just need some conversation / cutscenes added to flesh it out.
Ves - if you chose to help Roche save her that's another easy 'thanks for saving my life Geralt, please accept a steamy night in bed with me as repayment' storyline, which could easily lead to a more involved relationship like any others above.
Cool idea for a discussion, thanks for starting it.