So, you're saying Pokemon Colosseum isn't a Pokemon game? No, no, wait, you're saying that objectively Pokemon Colosseum isn't a Pokemon game?

It could be more but it also has to be careful to stay simple, a too complicated game could scare away new fans and too much change could isolate older fans away, at least in the main series. For now I think Pokemon can improve in the little things before going big, X and Y I thought had a rather lackluster story and the places in Kalos felt disconnected between each other.

How is the game I described not Pokemon?

You can trade literally every Pokemon that existed at the time to Colosseum, so I don't know what point you're trying to make. Almost every Pokemon game has unobtainable Pokemon. The difference between them and what you're suggesting is that one, those Pokemon still exist in that universe, and two, those missing Pokemon can be traded over so you can still use your favorites eventually. Pokemon is the poster child for a typical Pokemon spin off.

I agree, but I'm just describing a spin off. Also, the game I describe would still be simple. It would be like what Pokemon currently is, or what Smash is. It's deep once you know all the finer details, but is very accessable to beginners and children. 

So, it doesn't matter if those Pokemons are absent from the game itself as long as you can go and trade them?

What if "Pokemon Dark Days" feature a machine that can "clone" dead pokemon's DNA and import them into the game by trading them with your 3DS copy of...whatever Pokemon game you want to put here? It suddenly becomes Pokemon then? No, it suddenly becomes, objectively, a Pokemon game?

I'm sorry, but your idea of changing the core gameplay and still call it Pokemon is far less pokemon than my idea of just give it a setting which involves Pokemons dying. My opinion, though.

In that case risk is definitely reduced and GameFreak/Nintendo would have something to fall back on, the only issues now is cost since your idea sounds rather ambitious, and do not follow the current formula which is probably why I think Nintendo would be hesitant.  If they had a smaller project which was hugely sucessful then I think they would be more onboard.

1. An open-world Pokémon game, build around the same engine Ni-No-Kuni used, or perhaps an even better one. 

2. Release the game with the first 151 Pokémon.

3. Base it in the Kanto region (Red/Blue).

4. Don't use hack'n slash - use turn based combat. It has work for the series for many years.

5. Use expansions to open the world; add new regions, moves and Pokémon

6. THE MOST IMPORTANT ONE. Capture the beauty and feel of Pokémon.

The main games will always be strategy rpgs.  you will only see real time action on spin offs.

It doesn't matter if you can't catch a specific species in one particular game, as long as you can trade it over, yes. The fact that you don't get that shows your core misunderstanding of what Pokemon is. If you can suddenly clone every extinct species, then there is absolutely no reason to make a Pokemon game where they become extinct in the first place. So yes, that game would be objectively a Pokemon game, but it would be a pointless and confused one.

You're not just describing a game that kills Pokemon. XY is a game that kills thousands of Pokemon. You're describing a game that deletes the variety of Pokemon you can use from existance. If all you want is a Pokemon game that limites the roster during the main story campaign, you can play literally every single Pokemon game, but that's not what you were describing. You were describing a game that deletes the existance of half the Pokemon world because "more is worse." It's not.

Plenty of Pokemon games change the core gameplay and still feel like a Pokemon game. Unless it's a game like Pokken Fighters where the focus is solely on the 1v1 fighting, and no one story or relevancy to the Pokemon universe, you can't just take away Pokemon from Pokemon.