I suggest, as similar to others have said, that all pokemon are available in specific places all year round, but the changing seasons would bring changes in swarms and general pokemon appearance. I mean, does it really make sense for a wailmer to be found around a now-frozen lake :/?
I would appreciate different forms/pokemon only being available at specific times, yeah it would irritate the hell out of you, but it builds patience. I know some people who rip through the game without taking time to enjoy it :). I prefer to just take it easy. For example, ice types would be more common in winter, and grass types in spring and summer. And in summer cherrim could be in it's sunny day form permanently.