Story time, no TL;DR.
My wife was born in 95' and grew up never playing video games.
It wasn't until we started dating that she was exposed to them.

She got hooked on League of Legends for almost a year until World of Warcraft Classic was released and now that's pretty much all she plays now.
At some point while we were dating, I download Blue Version on my 3DS for her to try playing.
She collected her 3rd badge but ran into the problem where she didn't have flash and didn't know where to get it, after not being able to figure it out, she stopped playing it.

I'm convinced that she would love the 2nd and 3rd gens even more but getting her to complete a black and white game is difficult.
If she ever finishes Blue Version, what game should she move onto next?
I was thinking either Gold/Silver or Crystal but I'm just not sure if I should have her play through Gen 2 twice.

To me, it's unthinkable to not play through Gold or Silver and then Crystal because the story lines are slightly different but that might just be my nostalgia talking.