I’m already a librarian and love it.
Performing songwriter, which I already am but if I were younger I would go at this full time. It’s cool, by the way, to be recognized on the street sometimes.
Sushi chef.
Are you me? 
Day job (and night job, and weekends job, and holidays job): Freelance copyeditor. I’ve done a little bit of singing, but in folkie venues, and never where people paid admission specifically to hear me. I’d love to get into something a little more jazzy/bluesy/singer-songwritery, except I don’t play an instrument (at performance level) and I don’t write songs. So basically I need a songwriter and a band. Not too much to ask.
But the bit of singing I’ve done I’ve loved, and people do tell me I’m good at it. Who knows, maybe someday I’ll be able to parlay it into something more than singing for chump change and/or tips.
sighs wistfully
I’d be a park ranger at a beachside state park in Florida, ideally St. George Island but I’d be ok at St. Joe’s or others. I camp at these parks very frequently during the season and that always seems like just about the best job. They seem to basically just hang out, check in the campers, drive around once in a while and chat to campers, take their breaks on the beach, and then they go home. And they get paid for this.
My standard answer to this question for the last 45+ years is:
- Jet fighter pilot with aspirations to astronaut
- Emergency room physician
- Jazz vocalist
But now I want to add “Assistant to Cat Whisperer” at the top of the list. Ironically (or not) it’s the only one I’m remotely qualified for in this or any possible universe.
Barely finished high school after being kicked out at the end of 10th grade and couldn’t carry a tune in a Panamax oil tanker.
I changed my mind. Screw politics. I’d be Cecil Adams.
Radio talk show host/voiceover
(It’s actually what I went to college for)
Aircraft accident investigator. I’d love to work for the TSBC (though the NTSB statistically get more (and more interesting) cases, I don’t want to move to the US). I love following accident investigations, and read historical reports for fun and use them as a basis to teach myself as much as I can. It’s why I’m in school (for 2 more months…graduation, yeah!) and I hope to actually either attain this goal or be qualified enough in a sub-specialty to at least be called in as a technical advisor via ICAO A.13. I specifically want to work in commercial aviation; there’s something about a regional or jumbo jet that’s just fascinating to me!
Alright! I have staff! My first assignment for you is get us some snacks, then hang out on the internet until you’re finished. 
Movie screenwriter.
Or some kind of brains trust ideas guy who lazes around in a room full of paper darts and foosball tables, and throws out fantastic concepts that transform a movie or TV series into a runaway super mega hit.
Chief of Staff to the POTUS.
Park Ranger. I love being outdoors, working with my hands, and helping people out.
I would design video games.
It would be nice to convert my hobby of homebrewing into a job - i.e., running a brewpub or small brewery somewhere. I deeply fear that it would not be near as much fun to do for money, though. I’d settle for working for a really good craft brewery.
Foreign entertainer in Japan
POTUS
The second one is just a egotistical flight of fancy. The first one I’m actively gunning for.
I think that writing and music tend to be found together. More specificially I’m a technical writer, and I used to be a member of the Society for Technical Communication: the tech writer/musician combo is so common that when the annual convention was held in Nashville, there was an open mic night.
If I hit the lotto & had all my needs/bills handled, I’d still want to make people laugh if I could. I’d write jokes or lines or scenes…maybe some really bad poetry that would make school kids wince like someone just passed gas.
Or write a column somewhere on a regular basis that would spawn angry letters to the editor and uber-dramatic subscription cancellations.
“I’ll have you know that My Entire Bridge Club will No Longer Subscribe to ‘Towel Boy Monthly’ until his column is cancelled and he is Driven from the Earth…!”
Research scientist, for the joy of discovering something new and useful that would change people’s lives for the better.
Sizer at Victoria’s Secret.
Bestselling fantasy (urban/modern fantasy) author. Famous enough that I make a respectable living and could get paid/comped for conventions, but not famous enough that anyone beyond a pretty strong fan of my work would recognize me on the street.
If I didn’t have to worry about the cost of the years of additional education required, nor having to support myself in the process, I would love to be an Opacity Researcher for SETI. Barring that, an astronomer with unlimited dish time at the Keck Observatory.