First, congratulations. You have worked hard and achieved a lot, so enjoy it! Simply enjoying it makes it even better -- there are days I look out the window and think, how the fuck did I end up with this husband, with this family, with this job, with this house, etc. etc. etc. And even just having those thoughts gives me a glow for the rest of the day.
Second, you are asking the right question. You have met your basic needs and then some, you have accomplished the goals you set out to early on in life. So what now? Well, what is meaningful to you? If you've satisfied your ambition and money goals, maybe the next promotion or bigger house or whatever isn't what you need to strive for -- and kudos to you for recognizing that at this point instead of chasing achievement and bigger/better for the next 20 years. So what does matter? Spending time with your kid? Improving your physical health? Devoting more energy to your church, or to a charity that is meaningful to you?
Life can be a little bit of an oxymoron, because being so goal-oriented that you feel adrift without something else to achieve doesn't tend to breed real happiness; and yet at the same time, if we aren't stretching ourselves, growing, learning, being challenged, we wither away into meh-ness and boredom. I think the happy medium, one you have everything you need and most of what you want, is to focus on challenging yourself in different ways that are far more meaningful than just getting another raise or a better car or whatever.
And also: yeah, revisit this question in a few months. I have a sense some of that search for meaning may be, shall we say, subsumed by other, more pressing needs for a bit. ;-) So just enjoy where you are right now -- enjoy the hell out of still being the two of you, and the excitement of not knowing what comes next -- and then circle back to the bigger questions when you feel ready for them again.