… something utterly ridiculous.
Me?
I shall become a successful norteño singer. I will be known as El Gringo del Norte, and I will wear a red sequined suit and white cowboy hat. There are just three things I must do first:
Who’s next?
Get NASA to fund my Sex in Space study.
Design special device to help have sex in zero Gs.
Sell special device on late night TV with the tag line “designed by a NASA astronaut”.
??
Profit!
Get noticed by a racing team while I’m riding to work
Make hundreds of thousands of dollars as a star bicycle racer, beating guys ten years younger than me
Race in the Tour de France
I’m going to invent a device for men that translates what a woman says into what she means and suggests the response that is least likely to cause her to give him that look[sup]TM[/sup] (you know–the one every woman reading this is wearing on her face right now).
If I have time I’ll add a feature that reverses the process. A man enters a meaningful statement and the device cobbles together an appropriate string of words such that when a woman tries to decipher it (“I wonder what he really means by that”) she will inadvertantly stumble onto the actual meaning of the input statement.
Become one of the following:
God
Time-traveler
Pororoca Alien
Wonder who will catch this reference. 
May I point out that the “________ del Norte” naming convention becomes redundant when you append it to “El Gringo?” Gringos are “del Norte” by definition.
We now return you to your regularly scheduled unrealistic fantasy. 
Um…
So Seven, how’s that sex in space thing workin’ out?
Race in the Isle of Man TT. Win it.
THEN, get picked up by Yamaha or Ducati to race on their factory MotoGP team! 
Finally get noticed by some talent scout looking for the next hot heavy metal act, get a record deal, get rich and famous, then wait patiently for my eppy of “True Hollywood Story”.
Become famous for being famous. I’ll be on talk shows, appear in celebrity rags, be at all of the parties and cool places, and people will go “why is he famous?!”
(There’s just one snag. First I have to become famous).
ETA: A reality show would be nice, too.
I’m pretty sure that if I go to the driving range twice a week for the next few months, I’ll be able to pass Q-School and join the PGA Tour next year. I’ll enjoy that.
I will make love to every one of the Straight Dope Administrators, Advisory Board, and the entire Member Community by the end of 2007.
Twice.
OK, maybe I should have said every female member, but this way sounds slightly less creepy. Also, I think this Kleiner Klopfer has had an adverse affect on my Libido. 
I shall become pope.
No, actually it doesn’t
I will prove the existence, or non-existence of God, by that I mean a reference for every variant of God ever referred to in Theological debate, Philosophical argument, drunken discussions at the pub, etc. Then I will organize it in a sort of Wiki so atheists and theists can refer to it to save time and effort.
Hopefully, that will free up time to get the brilliant minds working on the real problems we face. Like how cold beer (and martinis) warms up too much too fast. How to keep a neat glass of scotch as cool as one on the rocks without all that nasty melting. And especially, get on that pesky problem of not having a world full of Olivia Munn clones.
I will create a new language named 0nqlîsh that will become the preferred choice of scientists, poets, novelists, and scholars.
I will use my new language to write the greatest novel known to mankind, dedicated to my first cat Pee Gee.
I will write, direct and star in the movie adaptation of my novel, which will become the highest grossing film of all time. Merchandising proceeds alone will make me a billionaire.
I will use the profits to research and develop nanotechnology robots capable of curing every known disease.
I will then oversee my own downfall, inventing a designer drug so powerful that upon my first test of it I’ll have a heart attack. My nanobots will repair my heart and save my life.
I will die at the age of 150 when, purely by chance, every atom in my body simultaneously and spontaneously ceases to exist.
My last words will be: “Rosebud”.
Cover of the Sports Illustrated Swimsuit Edition.
Mine is much more humble: I’ll lose 100 lbs by the end of the year. (nodnodnodnodnodnodnod)
Well, yes. But the Mexicans with whom I’ve discussed this idea think it’s hilarious. It’s part of the joke. I will also perform original songs that have risque lyrics, but perform them as if I think they’re perfectly innocent love songs. My first hit will be titled “Culo Veo, Culo Quiero”, a song about a boy with a crush on his pastor’s daughter.
I want to make first contact with an alien civilization. Following which, I will provide intelligence and guidance to our new alien masters as they conquer the planet. Ultimately, I’ll be appointed steward of the newly subjugated earth and will rule with great inconsistency and cruelty.