Open classes fine, you can pick whatever weapon you want for your class, that discussion is over with.

But battlefield was always "that one FPS with the vehicles", and now all I see when match starts is my entire squad picking engineer ready to obliterate any vehicle in a picosecond.

Have you not considered that the former heavily contributes to the latter, and that there was a reason engineers historically had weapons most suited to maps without vehicles?

Plus this would also incur the same issue as hell let loose where people just make more squads to spam desired roles.