Interesting question! Tbh, I'm actually really not sure how it would've gone. O.o Although I most certainly wouldn't be at Monash. Que massive list of all the many millions of things I was considering up to year 12~ (ignoring pre-year 10. I wanted to be a writer because I loved reading. Then I realised writing is hard lol)
Science was actually a late love for me - I despised it and maths up until about year 10, when I was more disgusted by it. I couldn't understand why anyone would do that kind of shit - the problem being that I was good at it. Less so science (probably because I did psych and am horrible at rote learning), but maths whenever I spoke to my teacher, she would just complain about my wasted potential. It got to the point that I used to hand in my tests early without doing the extended response question, because I couldn't be bothered with it and knew I'd already passed anyway. I did, however, enrol in methods because I could do it, and only needed 1/2 methods to do teaching, which at the time I was weighing up. As opposed to general, which I'd have to go all the way up to further to do if I wanted to do a teaching course. Also stats was my worst topic (actually failed it because of my laziness awks) and my teacher told me that methods was more algebra, which I could do fine.
Potentially teaching is where I'd go? I really enjoyed music and drama, and I definitely would've gone that way, but I was more interested in teaching as an after-thought, and would've done a masters in it. I knew I enjoyed helping others with things, but wanted to try other things first. However, I was limited by my lack-luster musical ability (brought on my lack of care for practice - I actually enjoyed the theory side a lot more than the performance side. I did enjoy the performance, but if I had to choose between reading about how the use of counterpoint was employed in Somebody That I Used to Know to commune with the listener to actually practising the song myself, I would've done the former). For this reason, I also very seriously considered being a composer, and was the only person to get an A+ in the past three or so years for an arranging assignment my teacher gave out every year, and even won a small competition once for one of my compositions (the output was a shit midi tho awks). I also liked Japanese and languages and thought about that, probably would've added to my teachable subjects if I went that path.
Then, I had to repeat year 10, because reasons that aren't academic that I don't want to go into. However, the school wouldn't let me repeat units, with the exception of units 1/2 Japanese (for the simple reason they wouldn't let me jump up to 3/4 in year 10 awks). Repeating 1/2 Jap destroyed my love of the language (still love learning about the culture, though, and try to keep what I do know in-tact), but I also decided to take chemistry and physics. Originally the school wouldn't let me do either of those because I chose them in year 9, but somehow managed to skirt the system with the repeat. I also took general maths advanced, because I refused to do year 10 maths again, but they wouldn't let me do methods (also I wanted to keep myself as much a year 10 as possible at the time, and my school didn't do methods in year 10, but would do GMA). My perceptions changed dramatically - I suddenly loved maths, and didn't mind science all too much. I still loved music and drama, though. At this point, I still thought education, but thought I might be a maths teacher instead of Jap to increase employability.
Then year 11 came along, and I was doing music, drama, finishing Jap, methods, English, and something else. I had no idea what to do - originally, my plan was lit, but I had decided I didn't enjoy reading anymore, so that was a no. But, a friend convinced me to do chem. I figured, it was alright in year 10 - I wasn't particularly enthralled, but she was a good friend, and we could suffer together. Then I had the best and most exciting teacher ever - she wasn't particularly special, but she very obviously cared. Whereas before if I ever got confused by something (because they simplified it too much and then something broke the rules), the teachers would ignore it, she'd go the extra step and explain why we simplified the thing we did, and why it broke down for this particular case. All of a sudden, science was this elegant, carefully balanced thing, with mystery and wonder. One problem - I didn't do biology, and couldn't find a career for just chemistry. My careers councillor was also no help ("Have you considered medicine?").
Then, I went on this wondrous camp just before year 12 (NYSF - if you're in year 10 reading about this, and like science, apply for it. You'll love it.), which opened my eyes to the world of science. The first thing it taught me was that I could do research (my current path), but there was something else it taught me about - the idea that I didn't have to go a uni in my state, and that there's also this large field in science communication that's poorly lacking.
So, the other way I would've gone after that was first and foremostly, another uni. I read. And read. And read. And holy shit, I knew so much about the HSC, SACE, the OP, etc. that I scared my friends - both interstate (because I made lots of those on NYSF) and in Victoria. Like, I wanted to know the systems inside out, so I knew what I was reading when I looked into UQ, UNSW, Curtin, UWA, ANU - everywhere. I even went on this massive "road-trip" with the little inheritance I got from my 18th birthday. I checked out UQ, QUT, Griffith, UNSW, USYD, UTS, Uni SA, UoA, Flinders Uni, UWA, Curtin, and some other uni in WA I've forgotten about, all in person, and had also learnt a bit about ANU at NYSF. I was set - in the end, I decided to do a Bachelor of Science (Advanced Maths) at UNSW, but also swapped with applied geophysics at UQ, geophysics at Curtin, and the PhB in ANU. I did look at Otago (NZ), but I needed HECS to get my degree, so quickly abandoned that idea.
However, I realised I didn't have the money to move interstate, and wouldn't be able to get any scholarships with my poor work ethic, and so decided to stick it out at Monash. The type of research I decided to do also swapped during the years, but I ended up here in chem, and haven't looked back.
The other thing I considered (which is actually still on the table) was to study a science communication degree, and ended up heavily looking at the Masters of Science Communication at ANU. This actually appealed to me because of my performing side - part of the degree entailed travelling around the country with the Questacon Science Circus, teaching the wonders of science to different schools and at various science programs during the year.
Tl;dr, I'm super indecisive and subject to change frequently lol, going from story writer, to teacher or composer, to several different types of teaching, to definitely not med and no clue, to science at like 5 different universities across the country, to research where I am now (swapping research fields monthly between now and start of uni), but might still go into science communication.