marshmallowcreampie
No matter what is going on in the story, the romance subplot will be THE most interesting thing to the readers, so make sure to put lots of focus on it, even when the fate of thousands is in the balance.
If you want your romance subplot to be even more exciting, after a while, put in another person to threaten the romance of the two characters and create a love triangle. Of course, the main character will still end up with the person that they spent the first 300 pages with. No reader will complain about it being predictable. The excitement of a love triangle doesn't come from the mystery of "Who will s/he choose?!", it comes from the reader enjoying the fantasy of having two people pine after him/her. Because everyone loves that!
To make your love triangle even more exciting, I recommend using the Hero as one side and the Villain as the other side. That makes it more
unpredictable on who the heroine will choose!
And if you really want to up the ante in your romantic plot, have the love interest get kidnapped by the villain. Double points if they don't even fight back.
Have your hero sabotage a marriage that would otherwise happen between the villain and their love interest. Readers love that stuff.
(( I have nothing against people who actually like wedding crashers. I like a good botched execution myself, so I'm somewhat guilty of this cliche. I will not let it go though. << ))