Some ideas for future generations:

Evolutionary stones:

1. An Ice/Water Fish that evolves with Ice Stone. This mimics the Water-types that evolved to Water/Ice species in Gen I, particularly Shellder.
2. A Steel-type that evolves into a Steel/Electric with Thunder Stone, Steel/Fire with Fire Stone, Steel/Water with Water Stone.
3. A Grass-type that evolves into Grass/Dark through Moon Stone, and Grass/Psychic through Sun Stone.
4. A species that has two gender exclusive evolutions, one Female and one Male, with one evolving through level up and other evolving with Dawn Stone. The common gender will evolve with the Stone, the rare gender will evolve through level.
5. A Psychic-type that evolves into Psychic/Dark through Dusk Stone, and Psychic/Fairy through Shiny Stone.
6. A Flying-type Eevee-esque species that evolves into variety of types through stones.
7. A Poison-type that evolves into a Poison/Water through Water Stone, and Poison/Grass through Leaf Stone.

Evolutionary moves:

1. A Pokémon that evolves with Cut into a Slashing species.
2. A Pokémon that evolves into a Fairy type with Covet.
3. A Pokémon that evolves into a Psychic-type with Confusion.
4. A Psychic Pokémon that evolves into different forms depending on the Screen move it has.

Evolutionary items:

1. A Water-type fish that evolves into a Water/Fairy through Sachet, and a Water/Steel through Metal Coat.
2. A Steel-type that evolves into a Steel/Fire by Magmarizer, and a Steel/Electric by Electivire.
3. A species that evolves depending on whether it holds Razor Fang or Razor Claw.
4. A Dragon-type that evolves depending on whether it holds Dragon Scale or Dragon Fang.

Marketable species:

1. A Psychic dog that evolves into different types depending on the game's version.
2. A Normal-type dog that has a different sub-type depending on the game's version. This dog would have a different primary type depending on its version it evolves.
3. A species that evolves into a different form depending on the region it was traded to.
4. A Dragon dog that evolves into different forms depending on the time of day with happiness.
5. A species that evolves depending on the stat boosting move it has.
6. A species that evolves with a special move tutor introduced in the secondary games of the generation.

Other gimmicks:

1.  A Species that evolves into different forms or species depending on its Ability. To have other abilities, it must use Ability Capsule, or the generation that introduces this idea rechecks Abilities for each evolution.
2. A species that evolves into a different type depending on its berry. The types are decided on Natural Gift's type table.
3. A species that evolves with a species of the same Egg Group as it. It would evolve into different forms depending on the Egg Group(and if it is the same Egg Group duo, it would be randomized.)
4. A species that evolves depending on IVs or EVs.
5. A species that evolves after obtaining a specific ribbon like Footstep or Hall of Fame ribbon.
6. A species that evolves after being hit by a specific type.
7. A species that evolves after an ability or move activates.
8. A species that evolves depending on how much of a stat it has.
9. A species that evolves with a species that has a specific ability or move.
10. A species that changes forms holding Deep Sea Scale or Deep Sea Tooth.
11. A species that changes depending on which Plate or Gem it holds.
12. A species that evolves depending on its location in the party.
13. A species that evolves with a species that has a specific stat distribution, like a stat above other, a stat a specific level, or a general orientation towards something.
14. A species that evolves depending on its nature.
15. A species that evolves with a Key item.
16. A species that evolves after encountering and or defeating a specific Pokémon.

Form changes:

1. A species that changes form after being traded over once.
2. A species that changes form after leveling up with an item.
3. A species that changes form after being traded over with an item once.
4. A species that changes form after being traded over once for a specific Pokémon.
5. A species that changes form after leveling up with max happiness.
6. A species that has a different form at hatching depending on Father's other Egg Group.
7. A species that changes form after encountering and or defeating a specific Pokémon.