I personally hope we kinda jump in with them being flirtatious and seeing it develop/happen, because it's been something building over two films. I think it would be pretty great to have that payoff and use it to tie into the overarching story. Kylo could use it against her, similarly to Vader using Leia against Luke (obviously not for romantic reasons).100%
Would love if they have an established relationship when the movie opens rather than go through a first kiss scene. Just a quiet, "hey, yeah we're seeing each other no big whoop" kinda vibe to it.
Seeing Rey flip her shit over Kylo taunting her with Finn's well being would be awesome to see, even if he's just bluffing.
You could do this non-romantically too, but I would love it to be done.