Black and White 1 has one of the most monotonous structures in the entire series, and it has some of the weakest ability to make good teams.
Essentially the structure of the game is: Gym, the counter is in a side route, Team Plasma encounter, repeat. I can even lay it out for you, this is changed only a few times.
N -> Monkeys in side route -> Gym 1 -> Team Plasma steals Pokemon -> Fighting Types in side route -> Gym 2 - > Team Plasma in forest -> Darumaka in bit of route up ahead -> Gym 3 -> N -> Side routes have good mons, though could find them on the way technically -> Gym 4 -> Cold Storage -> Peek up next route to get a Level 24 Deerling with Grass move and Jump Kick for Excadrill -> Gym 5 ->Chargestone has Team Plasma -> Emolga trade next to gym -> Gym 6 -> NO TEAM PLASMA -> Gym 7 -> Dragonspiral Tower (Team Plasma) -> Pawniard (Steel) in grass right before Dragon Gym -> Gym 8
While there are a few slight changes to the structure, BW1 very much works in a very predictable pattern even as a new player in my opinion. And it kind of renders teambuilding to be difficult. Keep in mind, before the first gym your only options are two Normal-Types (only one of which is worth a damn), a Dark-Type that sucks, and your elemental monkey, which also sucks after this gym. Then after that you get a few more options, but overall I find teambuilding in Gen 5 to be pretty dull and slow.
And I think it is telling that B2W2 and beyond very much try to keep a good pool of Pokemon in the region. And frankly, yes, I think keeping the dex to only the new Pokemon wasn't a good idea, it doesn't make teambuilding more creative, it turns it the opposite. While Gen 4 is infamous for having very similar teams, I think it is a game where it frontloads all of the good Pokemon, and the rest you have to very much go out of your way to get.
Meanwhile, the games I see have the most variety are the ones that give a lot of different, non-competing, viable options (as in, not going to make your playthrough more difficult) are the ones with the most variety. It doesn't even need to be like SWSH dropping like 150 Pokemon before the first gym, even a game like RSE has some good stuff. Mightyena really isn't terrible for an early/late game option, you can pick up a Lotad easily, Ralts, Wingull, etc. before the first gym. After that you get Mawile, Geodude, Sableye, etc.
In Gen 5, it follows a problem Gen 1 has. If there are similar typed Pokemon, they are usually version locked so you only actually have one, and so the variety suffers by sheer numbers. Gen 2 also.
You can say that it makes BW1 feel more special, but I'd say that making a game's design only really meaningful on a first playthrough (and that is debatable, again, the game has almost an exact type of team it wants you to craft, and basically leads you to it) in a series that got a lot of popularity (especially in the West) for being a very replayable RPG series, is in my opinion not a good idea. Especially with the workload needed to make it work.
Sun and Moon almost has this problem (early game can be a bit rough for options), but Akala Island is full of tons of variety of Pokemon. And then Ultra Sun and Ultra Moon becomes one of the most fun games to teambuild with in the entire series.
As for the story structure, me-no-like. It feels mostly arbitrary outside of the N fights, and I do not think we need anymore caves full of evil team members in Pokemon. Ever again. On replay, BW1 is also way more linear than people say. I think Alola and Unova BW1 have about the same amount of optional areas.
Like, you never even touch Alola's desert if you just follow the story, IIRC.
God, Gen 7 needs more appreciation.
