I would suggest they go to a play party, or just read more.
Loads of things inspire lust! Find that kernel, explore it in fantasy and use that openness/non-taboo to express what it is and/or how you’d like to explore it.
Anything can be a fun kink: silk ties, mirrors, feeding/getting fed, anonymous sex, fucking on the bonnet/hood of a vintage car on a countryside drive just before rain in early autumn, vanilla-as-kink, etc.
That’s the lovely thing about kinks, it’s just about being curious and open to explore what fancy comes to you. Some are keepers, some are best as fantasies, some aren’t logistically feasible, some aren’t timed right and some you just get bored of after a while.
Vanilla-as-kink is just what it sounds like: fetishising normative sex.
I’m not partaking myself, but I’d be willing to use it as part of a sadism play.
“I’m gonna tie you up and tease you within an inch of your life, and when you’re begging, screaming for more I’ll untie you and caress you softly… I’m going to kiss you twice, I’m going to penetrate you and cum inside you in just a couple thrusts, leaving you like that, utterly dissatisfied, and fall asleep next to you while you stew in your frustrations…”
“What are you gonna do to me tonight? I’m going to have us share a bottle of wine, grope you in front of the TV, I’m gonna drag you to the bedroom and fuck you, missionary style, with a semi stiff penis. You’re gonna smell the wine on my breath, my clumsy movements and flop under my mechanical thrusting, lulled by my meek grunts until I give up and fall down beside you, turning my back to you and fall asleep… snoring loudly.”
Or whatever vanilla folks do, I might’ve lost touch. There seems to be a lot of pent up frustration in those turning to the kinkier side though.