Think it will happen this generation? You get about 80 new Pokemon on the cartridge, 20 or so through FREE seasonal DLC, and then the obligatory event legendaries.
Not sure if the DLC Pokemon should be available for xx amount of time, or just stay available forever. An example being, oh my look, its winter! Free Delibird for you! Oh look, summer! Free Remoraid for you! And look, your Remoraid can evolve into a new Pokemon too, Octillery!! Halloween is coming, here's your free Misdreavus! If there were DLC Pokemon in GSC. lol
They shouldn't use DLC at all. It doesn't make sense to have DLC that you can trade to someone else's game.
Pokémon should be added through patches. The only reason siginificant free content isn't always added in patches is because they don't want to make the patches too big, but a single Pokémon on a 3DS game won't be that large.
So basically, DLC should be things that stick to one cart like new areas and stuff, while event Pokémon should be patched into everyone's game simultaneously.