There have been no major changes to battle mechanics, or overworld mechanics yet. All pokemon are intended to be available, except legendaries. Not even postgame. There is a large variety of pokemon available, even from the start, and all are intended to be viable. Though, this will take time.
Mightyena: Atk 90 -> 40 | Spatk 60 -> 120 | HP 70 -> 80
Onix: Def 160 -> 140 | Atk 45 -> 70 | Spd 70 -> 30 | Hp 40 (Only ability is sturdy, now is pure rock)
Steelix: Def 200 -> 190 | Atk 85 -> 100 | Spdef 65 -> 75 | spd 30 -> 1 | spatk 55 -> 65 (may hold tm 26 EQ)
Tyranitar: Atk 134 -> 140 | Def 110 -> 80 | Spatk 95 -> 130 | spdef 100 -> 80 | Spd 61 -> 80
Aggron: Atk 110 -> 90 | Hp 70 -> 100 | def 180 -> 160 | spatk 60 -> 70 | spdef 60 -> 80 | spd 50 -> 30 (will only have sturdy)
Wobbuffet: Hp 190 -> 200 | Atk 33 -> 255 | Def 58 -> 50 | Spatk 33 -> 255 | Spdef 58 -> 50 | Spd 33 -> 25 (Ability changed, i wonder what it is…)
Golem: Atk 110 -> 125 | def 130 -> 100 | spatk 55 -> 70 | spdef 65 -> 90 | Spd 45 -> 35
Gengar: Spatk 130 -> 120 | spdef 75 -> 55 | def 60 -> 55 | hp 60 -> 80 | atk 65 > 90
Blissey: spdef 135 -> 100 | Def 10 -> 40 | hp 255 -> 200
Sceptile: Atk 85 -> 50 | def 65 -> 75 | Spatk 105 -> 145 | spdef 85 -> 75 | Spd 120 -> 130 | hp 70 -> 60
Blazekin: Atk 120 -> 130 | Spatk 110 -> 130 | Def 70 -> 60 | SPdef 70 -> 60 | Hp 80 -> 60 | Spd 80 -> 100
Swampert: Hp 100 -> 110 | atk 110 -> 90 | def 90 -> 75 | spatk 85 > 135 | spdef 90 -> 65 | spd 60 -> 55
Raichu: Spd 100 -> 130 | spatk 90 -> 110
Sandslash: Hp 75 -> 80 | Atk 100 -> 125 | def 110 -> 90 | spdef 55 -> 65 | spd 65 -> 100
Ninetales: hp 73 -> 90 | atk 76 -> 70 | def 75 -> 80 | spatk 81 -> 100 | spdef 100 -> 85 | spd 100 -> 80
Vileplume: Hp 75 -> 80 | atk 80 -> 90 | def 85 -> 80 | spatk 100 -> 110 | spdef 90 -> 80 | spd 50 -> 70
Dugtrio: Hp 35 -> 40 | atk 80 -> 110 | def 50 -> 50 | spatk 50 -> 70 | spdef 70 -> 80 | spd 120 -> 140
Arcanine: hp 90 -> 130 | atk 110 -> 70 | def 80 -> 100 | spatk 100 -> 120 | spdef 80 -> 60 | spd 95 -> 85
Poliwrath: Hp 90 -> 70 | atk 85 -> 105 | def 95 -> 85 | spatk 70 -> 95 | spdef 90 -> 75 | spd 70 -> 85
Alakazam: Spd 120 -> 110 | spatk 135 -> 140 | spdef 85 -> 80 | hp 55 -> 60
Cloyster: Spdef 45 -> 55 | spatk 85 -> 100
gyarados: Atk 125 -> 60 | Spatk 60 -> 130 | spd 81 -> 90 | def 79 -> 60
Jolteon: HP 65 -> 50 | Atk 65 -> 75 | def 60 -> 50 | spatk 110 -> 120 | spd 130 -> 150
Sunflora: hp 75 -> 80 | atk 75 -> 70 | def 55 -> 50 | spatk 105 -> 135 | spdef 85 -> 90 | spd 30 -> 60
Rayquaza: I basically fucking mega evolve it lmao
Gardevoir: Spatk 125 -> 140 | hp 68 -> 69 | atk 65 -> 40 | def 65 -> 95 | spd 80 -> 35
castform: 100 across the board from 70
Houndoom: atk 90 -> 50 | spatk 110 -> 125 | def 50 -> 65 | spdef 80 -> 95 | hp 75 -> 90 | spd 95 -> 100
Forretress: + 15 across the board
unown : 80 across the board
slaking: Atk 160 -> 190
Banette: Hp 64 -> 80 | atk 115 -> 130 | spd 65 -> 80
Zapdos: +10 across the board (Has Drizzle, pure electric)