Rarity and Rainbow Dash;
Rainbow Dash ends up training hard with hardly any distractions without her mane six friend group, and gets through her wonderbolt training early and joins the wonderbolts at a young age. She is very skilled and well loved by fans as one of the fastest fliers with a handsome face. A real poster boy of a pony.
Rarity is a cantorlot based designer, being hired on by the wonderbolts exclusively when she was still early in her career looking for a hoof hold into a steady career. She designs their main outfits every year as well as outfits for special events like the gala and such.

One night Rarity and Rainbow Dash were making casual conversation at an event about the next look for the wonderbolts, and had papparazi capture a photo of them that made it look like they were being more than friendly with close whispers and walking off together to be alone (talking into each others ears then going outside because its too loud inside the building)
This leads to rumors about Rarity and Dash dating, which at first seems like a problem until sales for Rarity's products and Wonderbolt shows begins to go up. So on the Wonderbolts end up agreeing that the two should keep up this façade that their dating to the public for a little boost in popularity for both of them.
However after a few months of fake dating, the two end up learning more about each other and spending actual time together until they find themselves actually in love with each other.
Next thing you know the next big announcement at a Wonderbolt conference is Rainbow Dash announcing her engagement to Rarity.