Vin,
I also encourage you to submit Antarctica (and yes, w/ National Geographic), but $15K would only cover one person. But, only do that if Antarctica would be of interest to you. If you need any why's, you can search on this forum and find one of my old trip reports, or you can send me a PM. But, it would be a trip like none other, and if you are like us, when people ask what was your favorite trip of all time, you will likely say Antarctica. And (for Mark) while you would not make it all the way to the South Pole, you would likely cross over the Antarctic Circle (although there are no guarantees on an expedition...alll depends on weather).
Then after Antarctica, you will no doubt add the Artic Circle to your bucket list, and National Geo offers a Iceland/Greenland expedition, which is also awesome (and not as expensive...could come closer to squeezing in two people for your $15K prize. I can give you some good why's on that one, as we also took it a few years later.
But, what interests you? Where do you want to go? Do you know your heritage, where you could visit the country of your ancestors...meet present-day relatives there? (I did that in Ireland, and it was truly awesome finding a distant cousin I had never met, and she took us to the standing ruins of the cottage in which my grandmother and her multiple siblings were raised (I only had an old black/white photo from my childhood), saw family graves, church, school, and walk on the fields that were farmed by my great grandparents. That really was sort of spiritual. Different from Antarctica, but truly neat. Then add a night or two at Ashford Castle and do the Hawk Walk (Google if you have not seen that activity).
But, your best submission will truly be somewhere YOU really want to go for all the reasons YOU really want to go there. If you are motivated by your personal desires, likely your submission will be from the heart.
I'd love to go to Nepal, but that's just me.
My husband would love to see the Terra Cotta soldiers and the Great Wall in China (but I don't think we will be doing back to Asia, based solely on the really poor air quality we experienced last month in Vietnam and Hong Kong....it was really bad).
But, what is on YOUR list, and maybe some of us who may have been to those places can help you with some of the awesome things.
Hawaii was also somewhere we had never been, but oh my gosh....helicopter trips over a valcano and looking down into hot lava, then flying over the Napoli (sp?) Coast and seeing waterfalls that are 3500 feet high........that was awesome. You could definitely enjoy a lot of Hawaii for two on $15K and see some fun things. Walking thru an inactive lava tube (Thurston Lava Tube).
There are soooo many places to go see.
Buy the book "National Geographic's 500 Journeys of a Lifetime" or check it out of the library...........LOTS of inspiration in there. Mine is filled with sticky notes of places I've visited.
What a fun and wonderful thing your company is doing!!! Seriously wonderful.
I hope you win.