if you are worried about "balance" which no pokemon game in history of EVER has had,
Really undermining the work many people in the fandev community put in to their games, huh?
you are truly foolish and the only thing you really need to do is set a new encounter type and make it so most pokemon aren't available until after you beat the championships
So, lock 90% of pokemon in post-game, with no quality or thought put in to it?
Also, what about games that do not have championships or post-game? Or games that aim to tell a tailored story? Or games that take place in a specific region/period/for story reasons do not include XYZ pokemon? Do those also need to add all mons?
ANYBODY worried about a balanced Pokedex knows NOTHING about pokemon.
Anybody that solely wants all 900-ish mons in Pokemon to consider it a "good game", knows nothing about Pokemon either.
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P.S.:
I find it quite interesting how, in hindsight, I see little to no complaints about fangames not including all pokemon until SwSh came out, even though no official game ever included all pokemon without having the player resort to cheating, trading*¹ and transfering*¹. Most of said complaints are not because the fangame is boring or lacks something. It's because "GameFreak fucked up". GF's mistakes are not the communities to bare, nor should people be entitled to demand it from the community, since they cannot demand it from GF. If you want something done, do it yourself. I believe it is unfair to demand and insult others fandevs for not creating your ideal game. People have the same ease of tools as everyone else, there are no special requirements other than a Windows machine and a 2$ software to get you to make your game.
(kudos to you Lil Savage the Rapper for the big region, 16 badges, all mons and fakemon fangame that you're developing, you took it upon yourself. That, however, doesn't mean others have to do the same as you, nor that you have to do the same as others. They aren't foolish or lazy, they just have different goals than you do.)
*¹ - Trading and transfering are something that fangames cannot do easily, nor have the requirements to even justify wasting time to implement such things. Some games have done it, however many have not, nor need to. Most fangames have a playerbase of a few hundred at one particular moment in time before that number dwindles and fades. Few are those games, like Reborn or Uranium, that have thousands of players and a massive following. Do fandevs need to waste time implementing a feature that a very small group of people will ever use, rather than use their limited time developing other key aspects of a good videogame, such as balancing, story, graphics, animations, music, sound effects, stability, replayability, unique features, gameplay loop and more?