Evolution Revolutions?

Talking about Mega Evolutions reminds me of a different evolution issue, but one in the same vein. The issue concerns evolutions of regular Pokémon. Starting in the second generation, there was a tradition of providing older Pokémon with pre-evolutions (also called babies because they indeed often looked like immature versions of their evolutionary followups) as well as new subsequent evolutions. This tradition continued up until the fourth generation, and then suddenly stopped with the fifth generation onwards, except for a one-time return in the sixth generation to provide Eevee with Sylveon. Otherwise, after the fourth generation, there was nothing new in the way of these evolutions.

This state of affairs is similar to the one for Mega Evolutions, but it's one that has persisted longer. But unlike the issue for Mega Evolutions, there are two ways to think about this. On one hand, the absence of new evolutions for older Pokémon could mean that regular evolutions are now mature and only absolutely needed for Pokémon of the current generation - in other words, there is no longer any need to disrupt the evolutionary consistency of older Pokémon. On the other hand, some Pokémon have lore related to the concept of evolutions or transformations that have not been fully fleshed out. Eevee is certainly part of this way of thinking, since at present it and its evolutions only cover 9 out of the 18 types that are available. If Eevee is truly an evolutionary marvel, then it would have to cover all the types available sooner or later. Of course, what applies to Mega Evolutions also applies to regular evolutions, which means that if new evolutions do appear, their roles have to be considered in full.

Evolution is a core concept of Pokémon, whether in the regular ways or into Mega forms. That the regular way applies to most Pokémon of any single generation is something that's appropriate and expected. But as demonstrated by the second to fourth generations as well as the most recent Sylveon, the regular way can apply to provide retroactive linkages to a prior generation. The recent absence of this as well as new Mega forms may really just come down to their roles, whether or not they can be fulfilled considering existing Pokémon. The question of whether expansion can occur seems to still be an open one.