It doesn't matter if you're a novice or a pro. There will always be some pertinent questions to consider whenever our brain sparks with an idea we can't stop thinking about. While it may be tempting to jump headfirst into a book we're madly in love with, it's wise to stop and assess what we're about to do so we don't spend months (or years) locked into a project that's sapped away all that passion we had in the first place.
Who is this for?
This question has a couple layers to it. The first question is, am I writing this for me or for an audience (more on that here)? This will influence the way you write. The second layer (if you're writing for an audience) is to determine the target audience. Is it for adults? Children? Teenagers? Every age category has immutable guidelines about what's appropriate and what isn't. Your book will probably be more marketable if you stick to these guidelines.
What is this about?
If I were in an elevator with you and asked what your book was about, what would you say? You should be able to sum it up in less than a minute, especially if your idea is more high-concept or commercial.
Literary fiction is a little harder to boil down since it's more character-driven, but you should have enough of the components in place to be able to explain it coherently. If you can't, you probably have to do some more thinking on whether this idea will sink or swim.
Does this have novel-length potential?
I have a whole post dedicated to this topic here. In short, you have to look at the scope of the novel, the number of characters you have, word count guidelines by genre, and the time span of the story.
What am I trying to say?
In other words, what is the heart of the story? The theme, the moral, the lesson you are trying to convey. You may think this doesn't apply to you since you're toying with a light, rom-com-like idea, but every story has a moral, whether you intend to write one or not. It may be abundantly obvious or buried beneath the surface of your characters' thoughts, actions, beliefs, and decisions, which all have consequences. The other characters have reactions to these decisions.
No matter what you write, you are always saying something about family, marriage, gender roles, society, and culture (sometimes one of these things, sometimes all). How you, as a writer, deal with those issues reveals to readers what you think is moral and immoral, so think deeply about this one.
How is this story being told?
This one has to do with point of view, narrator, and tense. Whichever you pick (your possibilities are almost endless), whether it be first-person present tense, third-person past tense, or something else, keep in mind that the story will change according to the point of view. Think about it. Darryl telling a story about Karen will be different than Jeff telling a story about Karen, which will be completely different than Karen telling a story about herself!
You might not nail it on the first try. That's okay. If you have to go back after ten thousand words and change the narrator or tense, go for it. Just remember that there's no wrong choice. There may be a subjective "best choice" according to what you're trying to accomplish, but it's okay to explore your options before committing to something.
Shaelin, my favorite writer on YouTube, has a very comprehensive video on this.
I hope this is a good place to start. Good luck!