The start of the generation would always be defined by new Pokemon en masse, new starters, new cast of characters and a new region. However, what constitutes a generation seems to be more at liberty. We had a console switch (no pun intended), and a slew of new Pokemon in between generation starts. If a region was based on Australia, had a fire emu, a grass komodo dragon, and a water capybara, and features 80 new Pokemon with a professor that studies Pokemon populations, then that would be the start of new generation, and that is when the turnover happens.