Sorry for double posting.
When I was a 10 years old kid, instead of leaving home forever to catch'em all I used to think about... which pokemons could be nice in an hypotetical game. Most of "my game's" mons were fake, exept for a twenty-ish ones like Charizard or Deoxys, and the regional pokedex had 196 pokemons in total. I'll try to list as many as I remember, they were quite cool I think. Fakemons were inspired by my toys lol
If I had to create a game now, I would mostly think of what I thought as a kid. Have fun.
The games were pokemon Time and Fusion, not because of Dialga or Kyurem (bw2 wasn't even out yet) but because the legendary was basically my watch
So yeah the story was all about time, the villains were a Team Rocket-style serious crew who wanted power over the flow of time in order to sell it and basically dominate the world forcing everyone to buy time continuously.
The main legendary was indeed able to control the personal time, making time go faster or slower for everyone differently according to his will. He was good and had a zygardeish role in the ecosystem, but here comes the Fusoon legendary: a bouncy pink ball EHM a core of energy able to bond with any pokemon and influence it with his power. The villains were able to catch and make it dark, so he could basically make an any pokemon obey to the villains. In Time they alredy have it and their quest is to catch the watch, and vice versa in Fusion they alredy have Time's legend.
The game made great use of choices, you could basically interact with npcs in interesting ways and choose things like which towns you can access first according to how do you behave with others. You can also join the villains at any moment, it leads to a different storyline in which you want to become the leader and catch the legendary by yourself. After you do, most grunts will follow the old leader who tries things in the post game, but you still have a decent group of guys following you.
Speaking of postgame... my idea was to 1) make the game open world. The region had a slight hoenn inspiration, but you only get surf in the postgame. With surf however you can explore so many new areas making the map huge. There's also another region, made of few islands, where the plot continues and Deoxys plays a pivotal role in it
You also start to get boss battles in the story, battles with uncatchable forms of pokemons or with pokemons over lv100.
At the end of postgame you got a dark candy, a key item that could upgrade any lv100 mon to lv 101. It could be trained from there up to 200, the maximum level cap. But this was competitively useless and mainly for completionist
Aaaand yeah you get a battle frontier with eleven buildings. It had the 10 from the old games plus one unlocked after getting a silver in all the others - battle mirror. Just like tower, but all trainers have something from your team like a pokemon or someone with the same ability etc. The frontier brain is your mirrored image, with your same identical team but you are constantly slowed down on time. I'll get to time slowdown later.
There are also boss rush features a la bowser's inside story but that has nothing to do with battle frontier. It had huge rewards but I thought it to be almost impossible without lv200 mons
As for game mechanics, I added a new type - Cut type. Some moves (cut, slash, crush claw, air slash and a few more) became Cut type. Some pokemons also became Cut, like Scizor or Zangoose/Seviper. Or well Kartana, altough it didn't exist yet lol.
Another huge/small addition was attributes. Few pokemons had them, they were passive abilities linked to the species itself. They were things like "any received water damage is also an electric damage" or "non-stab moves get a 1.2 boost if the pokemon moves first". Not too impactful, but useful.
Also, there were Fusions. The legendary could bond to any pokemon making its secondary type Fighting, allowing him to use aura sphere and vacuum wave, and giving him a special attack boost in base stat if it was low. Other stats could change as well. As legends were untradable, only one fusion pokemon was allowed in a team, and it counted as an uber slot
We have then Time flows, the ultimate form of speed control. There were four forms of the legend, normal attack defens and tech, with different time-changing moves. Normal had good stats and time reset, which negated any buff/drop to speed, tailwind or trick room or anything else from both sides of the field. Just an honest pokemon, with the best stats of the forms. They all were dark-bug btw because yes. Attack was a glass cannon with time speeder, that doubled its time for three turns after the one the move was used, meaning he could attack twice in the same turn for three times selecting two different moves. Using the move however dropped significantly the alredy low defenses so it was risky. Defense was my favourite. Bulky but not passive, time slowdown halved opponent's speed for 3 turns after use and negated trick room during the effect. Tech finally had balanced stats. Weirdtime, for 3 turns either player could attack twice with the same pokemon at the cost of not attacking with the other. Just for doubles.
Deoxys could also learn those moves but they were less effective for him
I also had A TON of minor changes in mind like a new kind of secret bases, or new ways to play online but I'm tired and I'll talk about it later. Alongside my pokemons, I still remember everyone