Karpi

So, the new starter pokemon are pretty cute - the best set we've had since Hoenn imo. But that's not how I wanted them to turn out. I wanted the starter pokemon to have secretly creepy backstories that you discover as you play through the game. And I've had plenty of ideas, so here are some of my favorites:

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It starts out cute enough, perhaps looking like this:

And you, as the pokemon trainer, think "that's a cute little staff, maybe it will learn some good coverage moves later on"
And as you level up, it does learn various moves like Leech Life, Drain Punch, and Psycho Shift in addition to the regular grass type move like Absorb and Leech Seed, so it seems like a win-win situation.

Once it evolves, the pokemon gains a new ability that boosts its attack stat by 1 at the end of every turn in exchange for stealing a health point from one of its teammates. This isn't really a big deal, since your Graveler with 67 remaining HP is just as useful as one with the full 68 HP.

But then you reach its final form, and the forest reaper finally emerges. You discover that he has immense power, but your teammates appear to die at random times.
Tree Monster by *Guesscui on deviantART
The forest reaper has the Leech Power attack, your greatest weapon. It adds its attack stat to that of one of your random teammates, and then attacks the opponent with the combined attack stats and a base power of 100. The drawback? Your teammate loses the same amount of health as the opponent. Because the forest reaper must suck power from its mindless followers in order to maintain its dominance over the woods
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But, you know, you ask dear old Professor Willow about the grass type, and he says he's heard some strange rumors of zombie-like behavior from the trainers that picked that one. But not to worry! he reassures you, the fire starter comes straight from a lab and thus any worries of voodoo magic should be nonexistent!

Plus it's cute:

It starts out with the moves Ember and Smog, but you don't really think anything of it at the time because, after all, Magmar learned Smog. It soon learns Acid and Fire Spin, and you think "neat, now that creepy evil grass starter will be twice as scared of me!"

And when it evolves, it only becomes more powerful. It looks slightly deformed at this point, and has a new ability that causes its stats to randomly fluctuate during battle. This is sort of a nuisance, but it does comparatively well to the rest of your team. Also, sometimes a random status effect will appear on the starter of some of its teammates.

Then, the final evolution. It comes a few levels earlier than the other starters, so your first reaction, naturally, is "neat!"
No. Not neat in the least. This feeling of elation disappears when you see its face:

It has a damn high special attack stat, though, but you had better use it quick.
Now it becomes apparent that the random fluctuating in stats was attached to a decay equation, and every pokemon on your team appears to suffer from it. This is the result of the heavily irradiated fire type that scientists unsucessfully tried to merge fungal genes with.

All your pokemons' stats become worse as time goes on and they succumb to the radiation poisoning. If only you had not chosen the fire starter.
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So you, as a local of the generically named small town where Professor Willow lives, have seen the soulless husks of a person walking down the street as their grass starter devours the energy of those around it, and you have seen trainers with the fire starter become progressively more sick until they finally just collapse into a twitching mess on the ground and die.

As a well-educated 10 year old, you therefore choose the water starter, regardless of its unusual appearance:

Its longevity contrasts sharply with that of the fire starter. In fact, this pokemon species has been around for thousands of years. It starts out with Ancientpower, Brine, and several other powerful moves. Professor Willow warns you that the pokemon will be intent upon reproducing once it evolves fully, which confuses you when your pokedex identifies it as a genderless pokemon.

Upon evolution, it learns the dangerous move Destiny Bond, as if daring the opponent to try and kill this pokemon, whose ability grants it a numerical bonus to its stats that increases exponentially with its level. There is really no reason to use your other pokemon, even at this middle evolution stage. You can't wait for it to evolve and want to reproduce, because with more than one of these, no trainer alive could defeat you!

When this pokemon reaches its final form, your expectations of its epicness are fully met with a terrifying fossilized sea monster:

In this fully mature stage, the pokemon wants to reproduce, but you soon discover that its method of reproduction is, strangely, asexual. Half of its life energy enters into your body, and a satanic voice in your head explains that upon your death, you will be transformed into the starter pokemon you picked at the beginning of your journey.

You have finally learned how this ancient species has propagated itself for so many years. Now you have only to spend the rest of your mortal life crushing your opponents as a great pokemon trainer, with the uneasy thought that afterward, for eternity, an endless chain of pokemon trainers will be commanding you to do the crushing.
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So, I know some of my ideas are pretty dumb, but those are the starters I would put into a pokemon game! Personally, I would choose the grass starter... I think.