just don't tell the fast mover guys (well, maybe harriers aren't all that fast) that you think their platform is the perfect stepping stone to that G4.
anything you fly in the Marines will be good and I suspect you can go to the more generic airplanes from any of them. We had pilots from every Navy platform that I could think of (plus some Army guys and even a Marine) flying C40s in my reserve squadron. I'm Navy and it was a Navy squadron but I suspect that Marine executive lift is pretty similar.
I'd put your goals in this order and priority.
1- finish OCS
2 - finish flight school
3 - aim for the platform of your choice in flight school but realize that it comes down to A - needs of the Corps, B- how you did in primary flight training, and finally, C - what you want.
4 - excel in your first fleet squadron, be it helos or jets or fixed wing. that will give you options when it comes time to choose your follow on tour.
it will be fun and hard and you will enjoy it. and like a previous poster said, every Marine is a rifleman first. So, even if you do go pilot, you'll get plenty of mud and shooting during TBS.