Catching up. I'm kinda salty I missed the difficulty discussion, but I gotta address another post first.
...Which brings the question of why not try to rebalance mons?
Impossible. Matter of fact, it has been since at least Gen 2 and I can personally confirm this having attempted to do so hacking Stadium 2.
You can make a couple adjustments while trying to not step on another mon's niche, but even then you need to immediately recognize not all mons are going to be winners. At best you'll be able to give them an identity and hope they'll be fun in the lower tiers.
Apparently there seem to be some stuff from Platinum that may be in the game like Porygon-Z who isn't obtainable even in the post game and an NPC that may give you the item to change Shaymin's form.
Pory-Z can be obtainable in DP at the Trophy Garden, it's just post-game. (Unlike in Platinum, Mr. Backlot doesn't run his mouth until you get the Nat. Dex)
Obviously though, it's a potential clue that the Sinnoh Dex is in fact, expanded from the abysmal DP Dex.
I don't think people necessarily want high difficulty in Pokémon. I've seen many posts about how that isn't really feasible given the current battle mechanics (free +100% damage bonus if you can identify the typing of the opponent and have the right move for it being the greatest example), static enemy teams, and the symmetrical design philosophy (anything you can face, you can also use - which Pokémon recently seems to have distanced itself from). The player simply has too many advantages. High difficulty is hard to create, and it probably wouldn't be very fun all in all either.
But people want a step up from a "poke the enemy and it keels over" level of difficulty too. There is something about seeing late-game Gym Leaders with three Pokémon with several empty moveslots, a total lack of route Trainers carrying more than two Pokémon, held items almost never being used, or a conspicuous lack of anything resembling strategy from in-game opponents. As a player, you don't feel like you're being challenged on even terms, Instead, the game deliberately restrains itself from battling you on your level. The rival deliberately picking the starter weak to yours is a great example of this. It's as if the game bends over to facilitate your victory, and it's being really obvious about it. That's frustrating. It's the difference between being challenged and being led to victory. Of climbing a wall versus going up an escalator.
Arguably, the Challenge Mode in BW2 doesn't make the game that much harder. Only a few trainers are affected by the setting, and them having higher-leveled Pokémon even means you're getting more XP to fight the common route trainers with. But still it feels good to see the Gym Leader having an extra Pokémon, with moves to address the glaring weaknesses of their specialty type. Or Elite Four teams full of held Items. It's the game letting itself play a little smarter, letting you face a higher level of challenge. It's still not particularly difficult, but it's not "Oh no, here comes the player, better do everything we can to let them win!" And that makes a world of difference.
The games don't have to be hard to be fun. But I'd say they need to be less in-your-face easy.
There's actually a very simple way to make the battles at least more interesting.
Thematic team building. Game Freak is no stranger to the idea, even in modern times, Kabu is a perfect example of what I mean.
A huge problem with Rom Hacks is that they often fall into the obvious pitfalls of stupid optimized major battles, fully evolved mons when the player doesn't have access to good tools, or alternatively a bunch of pseudos everywhere late-game, level spikes everywhere, and crazy level inflation.
You'll notice that all of these usually promote mindless grinding so that the player can actually push through those. No strategic value whatsoever.
Now let's go back to Kabu. Kabu does have some interesting tricks up his sleeve. Instead of going with a basic hyper offense team, as expected of Fire trainers, his team is designed around spreading burns with Will-O-Wisp.
The result? His team becomes much more interesting because there's a strategic flavor involved. You can't just pull up with any old Rock-type and expect an easy sweep, matter of fact, even a mon like Drednaw, otherwise a hard counter, doesn't want to take a Will-O-Wisp.
Naturally, Game Freak pulled their punches too much and made him a wimp anyway, but there's still value in that approach because they gave Kabu an identity that other leaders, for example, Flannery, didn't have.
Flannery is the exact opposite, her identity is much more in line with the hyper offensive Fire specialist I mentioned earlier, but she still manages to stand out because of her tactic of combining Overheat (and at mid-game that thing is a serious concern) and a White Herb to remove Overheat's drawback.